virtio: rng: split out common code in probe / remove for s3/s4 ops

The freeze/restore s3/s4 operations will use code that's common to the
probe and remove routines.  Put the common code in separate funcitons.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>