vt/vga: ignore ACPI_FADT_NO_VGA unless running virtualized

There's too many broken hardware out there that wrongly has the
ACPI_FADT_NO_VGA bit set. Ignore it unless running as a virtualized
guest, as then the expectation would be that the hypervisor does
provide correct ACPI tables.

Reviewed by: emaste, 0mp, eugen
MFC: 3 days
Sponsored by: Citrix Systems R&D
PR: 230172
Differential revision: https://reviews.freebsd.org/D34392