Add VirtIO console driver

Support for the multiport feature is mostly implemented, but currently
disabled due to some potential races in the hot plug code paths.

Requested by:	marcel
MFC after:	1 month
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273515 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f