vt: Use textmode when we're running on hypervisors

The graphic mode is noticeably slow on hypervisors, especially
on Hyper-V (1 second to each line).

Submitted by:       Dexuan Cui <decui microsoft com>
Reviewed by:        kib, sephe, royger (early loader version)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5739