qemuMigrationBeginPhase: Check for 'drive-mirror' for NBD

So far we are checking if qemu supports 'nbd-server-start'. This,
however, makes no sense on the source as nbd-server-* is used on the
destination. On the source the 'drive-mirror' is used instead.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_migration.c

@@ -1987,7 +1987,7 @@ static char
         goto cleanup;
 
     if (flags & (VIR_MIGRATE_NON_SHARED_DISK | VIR_MIGRATE_NON_SHARED_INC) &&
-        virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_NBD_SERVER)) {
+        virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_DRIVE_MIRROR)) {
         /* TODO support NBD for TUNNELLED migration */
         if (flags & VIR_MIGRATE_TUNNELLED) {
             VIR_WARN("NBD in tunnelled migration is currently not supported");