qemu: Refactor snapshot check for _LIVE vs. _REDEFINE

The current qemu code rejects the combination of the two flags
VIR_DOMAIN_SNAPSHOT_CREATE_LIVE in tandem with
VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE, but rather late in the cycle
(after the snapshot was already parsed), and with a rather confusing
message (complaining that live snapshots require external storage,
even if the redefined snapshot already declares external storage).
Hoist the rejection message to occur earlier (before parsing any
XML, which also aids upcoming patches that will implement bulk
redefine), and with a more typical error message about mutually
exclusive flags.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -15696,6 +15696,9 @@ qemuDomainSnapshotCreateXML(virDomainPtr domain,
     VIR_REQUIRE_FLAG_RET(VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE,
                          VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY,
                          NULL);
+    VIR_EXCLUSIVE_FLAGS_RET(VIR_DOMAIN_SNAPSHOT_CREATE_LIVE,
+                            VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE,
+                            NULL);
 
     if ((redefine && !(flags & VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT)) ||
         (flags & VIR_DOMAIN_SNAPSHOT_CREATE_NO_METADATA))
@@ -15756,8 +15759,7 @@ qemuDomainSnapshotCreateXML(virDomainPtr domain,
     /* reject the VIR_DOMAIN_SNAPSHOT_CREATE_LIVE flag where not supported */
     if (flags & VIR_DOMAIN_SNAPSHOT_CREATE_LIVE &&
         (!virDomainObjIsActive(vm) ||
-         def->memory != VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL ||
-         redefine)) {
+         def->memory != VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL)) {
         virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
                        _("live snapshot creation is supported only "
                          "during full system snapshots"));