bhyve: Use vm_get_highmem_base() instead of hard-coding the value

This reduces the coupling between libvmmapi (which creates the highmem
segment) and bhyve, in preparation for the arm64 port.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40992