bhyve: avoid allocating BARs above the end of supported physical addresses.

Read CPUID leaf 0x8000008 to determine max supported phys address and
create BAR region right below it, reserving 1/4 of the supported guest
physical address space to the 64bit BARs mappings.

PR: 250802 (although the issue from PR is not fixed by the change)
Noted and reviewed by: grehan
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D27095