x86/xen: do video console fixup as part of early initialization

When FreeBSD is running as dom0 the video console metadata provided by the
bootloader might not be accurate, as Xen has very likely taken over the console
and possibly changed the mode.

Adjust the video console information in the kernel metadata as part of early
Xen initialization.

Sponsored by: Cloud Software Group
Reviewed by: imp
Differential revision: https://reviews.freebsd.org/D43934