qemu: disable external snapshot of readonly disk

Disable external snapshot of a readonly disk for domains as
this operation is not very useful. Such a snapshot is not
possible for active domains but the error message from QEMU
is more cryptic:

     error: internal error: unable to execute QEMU command 'transaction':
                         Could not create file: Permission denied

This error at least makes the error more understandable for
active domains and disallows for inactive domains as well.
Signed-off-by: NNikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_driver.c

@@ -14755,6 +14755,13 @@ qemuDomainSnapshotPrepareDiskExternal(virDomainDiskDefPtr disk,
     int ret = -1;
     struct stat st;
 
+    if (disk->src->readonly) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                       _("external snapshot for readonly disk %s "
+                         "is not supported"), disk->dst);
+        return -1;
+    }
+
     if (qemuTranslateSnapshotDiskSourcePool(snapdisk) < 0)
         return -1;