doc: Be more specific about semantics of _REUSE_EXT flag

Snapshots and block-copy have a flag that forces qemu to re-use existing
file. Our docs weren't exactly clear on what the existing file should
contain for this to actually work.

Re-word the docs a bit to state that the file needs to be pre-created in
the desired format and the backing chain metadata needs to be set prior
to handing it over to qemu.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1084360

src/libvirt.c

@@ -18394,10 +18394,12 @@ virDomainSnapshotGetConnect(virDomainSnapshotPtr snapshot)
  * destination files already exist as a non-empty regular file, the
  * snapshot is rejected to avoid losing contents of those files.
  * However, if @flags includes VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT,
- * then the destination files must already exist and contain content
- * identical to the source files (this allows a management app to
- * pre-create files with relative backing file names, rather than the
- * default of creating with absolute backing file names).
+ * then the destination files must be pre-created manually with
+ * the correct image format and metadata including backing store path
+ * (this allows a management app to pre-create files with relative backing
+ * file names, rather than the default of creating with absolute backing
+ * file names). Note that setting incorrect metadata in the pre-created
+ * image may lead to the VM being unable to start.
  *
  * Be aware that although libvirt prefers to report errors up front with
  * no other effect, some hypervisors have certain types of failures where
@@ -19864,11 +19866,14 @@ virDomainBlockPull(virDomainPtr dom, const char *disk,
  * by adding VIR_DOMAIN_BLOCK_REBASE_COPY_RAW to force the copy to be raw
  * (does not make sense with the shallow flag unless the source is also raw),
  * or by using VIR_DOMAIN_BLOCK_REBASE_REUSE_EXT to reuse an existing file
- * with initial contents identical to the backing file of the source (this
- * allows a management app to pre-create files with relative backing file
- * names, rather than the default of absolute backing file names; as a
- * security precaution, you should generally only use reuse_ext with the
- * shallow flag and a non-raw destination file).
+ * which was pre-created with the correct format and metadata and sufficient
+ * size to hold the copy. In case the VIR_DOMAIN_BLOCK_REBASE_SHALLOW flag
+ * is used the pre-created file has to exhibit the same guest visible contents
+ * as the backing file of the original image. This allows a management app to
+ * pre-create files with relative backing file names, rather than the default
+ * of absolute backing file names; as a security precaution, you should
+ * generally only use reuse_ext with the shallow flag and a non-raw
+ * destination file.
  *
  * A copy job has two parts; in the first phase, the @bandwidth parameter
  * affects how fast the source is pulled into the destination, and the job

tools/virsh.pod

@@ -876,10 +876,11 @@ flattens the entire chain; but if I<--shallow> is specified, the copy
 shares the backing chain.
 
 If I<--reuse-external> is specified, then I<dest> must exist and have
-contents identical to the resulting backing file (that is, it must
-start with contents matching the backing file I<disk> if I<--shallow>
-is used, otherwise it must start empty); this option is typically used
-to set up a relative backing file name in the destination.
+sufficient space to hold the copy. If I<--shallow> is used in
+conjunction with I<--reuse-external> then the pre-created image must have
+guest visible contents identical to guest visible contents of the backing
+file of the original image. This may be used to modify the backing file
+names on the destination.
 
 The format of the destination is determined by the first match in the
 following list: if I<--raw> is specified, it will be raw; if
@@ -3172,7 +3173,8 @@ metadata again).
 
 If I<--reuse-external> is specified, and the snapshot XML requests an
 external snapshot with a destination of an existing file, then the
-destination must exist, and is reused; otherwise, a snapshot is refused
+destination must exist and be pre-created with correct format and
+metadata. The file is then reused; otherwise, a snapshot is refused
 to avoid losing contents of the existing files.
 
 If I<--quiesce> is specified, libvirt will try to use guest agent
@@ -3233,8 +3235,9 @@ results in the following XML:
 
 If I<--reuse-external> is specified, and the domain XML or I<diskspec>
 option requests an external snapshot with a destination of an existing
-file, then the destination must exist, and is reused; otherwise, a
-snapshot is refused to avoid losing contents of the existing files.
+file, then the destination must exist and be pre-created with correct
+format and metadata. The file is then reused; otherwise, a snapshot
+is refused to avoid losing contents of the existing files.
 
 If I<--quiesce> is specified, libvirt will try to use guest agent
 to freeze and unfreeze domain's mounted file systems. However,