qemu: snapshot: Improve detection of mixed snapshots

Currently we don't support mixed (external + internal) snapshots. The
code detecting the snapshot type didn't make sure that the memory image
was consistent with the snapshot type leading into strange error
message:

 $ virsh snapshot-create-as --domain VM --diskspec vda,snapshot=internal --memspec snapshot=external,file=/tmp/blah
 error: internal error: unexpected code path

Fix the mixed detection code to detect this kind of mistake:

 $ virsh snapshot-create-as --domain VM --diskspec vda,snapshot=internal --memspec snapshot=external,file=/tmp/blah
 error: unsupported configuration: mixing internal and external targets for a snapshot is not yet supported

src/qemu/qemu_driver.c

@@ -12755,7 +12755,9 @@ qemuDomainSnapshotPrepare(virConnectPtr conn,
     /* For now, we don't allow mixing internal and external disks.
      * XXX technically, we could mix internal and external disks for
      * offline snapshots */
-    if (found_internal && external) {
+    if ((found_internal && external) ||
+         (def->memory == VIR_DOMAIN_SNAPSHOT_LOCATION_INTERNAL && external) ||
+         (def->memory == VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL && found_internal)) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                        _("mixing internal and external targets for a snapshot "
                          "is not yet supported"));