Improve error handling in do_snapshot_blkdev()

In case we cannot open the newly created snapshot image, try to fall
back to the original image file and continue running on that, which
should prevent the guest from aborting.

This is a corner case which can happen if the admin by mistake
specifies the snapshot file on a virtual file system which does not
support O_DIRECT. bdrv_create() does not use O_DIRECT, but the
following open in bdrv_open() does and will then fail.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
