Support PCI extended config space in bhyve.

Add the ACPI MCFG table to advertise the extended config memory window.

Introduce a new flag MEM_F_IMMUTABLE for memory ranges that cannot be deleted
or moved in the guest's address space. The PCI extended config space is an
example of an immutable memory range.

Add emulation for the "movzw" instruction. This instruction is used by FreeBSD
to read a 16-bit extended config space register.

CR: https://phabric.freebsd.org/D505
Reviewed by: jhb, grehan
Requested by: tychon