vt(4): Add support to "downgrade" from eg. vt_fb to vt_vga

The main purpose of this feature is to be able to unload a KMS driver.

When going back from the current vt(4) backend to the previous backend,
the previous backend is reinitialized with the special VDF_DOWNGRADE
flag set. Then the current driver is terminated with the new "vd_fini"
callback.

In the case of vt_fb and vt_vga, this allows the former to pass the
vgapci device vt_fb used to vt_vga so the device can be rePOSTed.

Differential Revision: https://reviews.freebsd.org/D687