rbd: Assign s->snap/image_name in qemu_rbd_open()

rbd: Assign s->snap/image_name in qemu_rbd_open()

Now that the options are already available in qemu_rbd_open() and not
only parsed in qemu_rbd_connect(), we can assign s->snap and
s->image_name there instead of passing the fields by reference to
qemu_rbd_connect().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>