Allow us to use the virtual timer. It is currently disabled, but should
be usable as the default timer in place of the physical timer.

We are guaranteed to have access to the virtual timer, but when running
under a hypervisor may not have access to the physical.

Differential Revision: https://reviews.freebsd.org/D588


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271189 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f