bhyve: add more slop to 64 bit BARs

Bhyve allocates small 64 bit BARs below 4 GB and generates ACPI tables
based on this allocation. If the guest decides to relocate those BARs
above 4 GB, it could lead to mismatching ACPI tables. Especially
when using OVMF with enabled bus enumeration it could cause
issues. OVMF relocates all 64 bit BARs above 4 GB. The guest OS
may be unable to recover from this situation and disables some PCI
devices because their BARs are located outside of the MMIO space
reported by ACPI. Avoid this situation by giving the guest more
space for relocating BARs.

Let's be paranoid. The available space for BARs below 4 GB is 512 MB
large. Use a slop of 512 MB. It'll allow the guest to relocate all
BARs below 4 GB to an address above 4 GB. We could run into issues
when we exceeding the memlimit above 4 GB. However, this space has
a size of 32 GB. Even when using many PCI device with large BARs
like framebuffer or when using multiple PCI busses, it's very
unlikely that we run out of space due to the large slop.
Additionally, this situation will occur on startup and not at runtime
which is much better.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33118