bhyve: Drop volatile qualifiers from snapshot code

They accomplish nothing since the qualifier is casted away in calls to
memcpy() and copyin()/copyout(). No functional change intended.

MFC after: 2 weeks
Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D37292

(cherry picked from commit 8b1adff8bcbdf0e58878431c6ed5a14553178d4d)