MFC 297334

MFC 297334

    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

git-svn-id: svn://svn.freebsd.org/base/stable/10@301950 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f