Advertise 64-bit physical-address capability.

This fixes a coredump with NetBSD guests when XHCI is configured.
On seeing the AC64 flag clear, the NetBSD XHCI driver was only writing
to the lower 32-bits of 64-bit physical address registers. The emulation
relies on a write to the hi 32-bits to calculate a host virtual address
for internal use, and has always supported 64-bit addressing.

All other guests were seen to write to both the lo- and hi- address
registers, regardless of the AC64 setting.

Discussed with: Leon Dang (author)
Tested with: Ubuntu 16/18/20, Windows10, OpenBSD UEFI guests.

MFC after: 2 weeks.