bhyve/snapshot: split up mutex/cond initialization from socket creation

Move initialization of the mutex/condition variables required by the
save/restore feature to their own function.

The unix domain socket that facilitates communication between bhyvectl
and bhyve doesn't rely on these variables in order to be functional.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D30281

(cherry picked from commit fdbc86cf79784f56fab8115f2d565962e1111b2e)