bhyve: move 64 bit BAR location to match OVMF assumptions

OVMF will fail, if large 64 bit BARs are used. GCD-Map doesn't cover
64 bit addresses of BARs.
OVMF assumes that 64 bit addresses of BARS are located on next 32 GB
boundary behind Top of High RAM.

This patch moves 64 bit BARs on next 32 GB boundary behind Top of High
RAM to match OVMF assumptions.

Differential Revision: https://reviews.freebsd.org/D27970
Sponsored by: Beckhoff Automation GmbH & Co. KG