block: acquire in bdrv_query_image_info

NFS calls aio_poll inside bdrv_get_allocated_size.  This requires
acquiring the AioContext.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1450867706-19860-1-git-send-email-pbonzini@redhat.com
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>