bhyve: Remove an unneeded vm_get_register() call in main()

At one point the RIP value was passed to fbsdrun_addcpu(), but this is
no longer the case. No functional change intended.

Reviewed by: jhb, corvink
Sponsored by: Innovate UK
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40988