block: Classify bdrv_get_flags() as I/O function

This function is safe to call in an I/O context, and qcow2_do_open()
does so (invoked in an I/O context by qcow2_co_invalidate_cache()).

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220427114057.36651-2-hreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>