block: early check for blockers on drive-mirror

block: early check for blockers on drive-mirror

Even if an op blocker is present for BLOCK_OP_TYPE_MIRROR_SOURCE,
it is checked a bit late and the result is that the target is
created even if drive-mirror subsequently fails.  Add an early
check to avoid this.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>