Change NO_EVENTTIMERS from an arm-specific to an MI option, so that it can
be used in MI code.

This is intended as a temporary measure to unbreak the build.  The real fix
is to write event timer drivers for legacy arm hardware, then get rid of
this option completely.  That's going to take a few days.
Add support for the virtio RNG entropy-source device.
Call through to /dev/random synchronously to fill
virtio buffers with RNG data.

Tested with FreeBSD-CURRENT and Ubuntu guests.

Submitted by:	Leon Dang
Discussed with:	markm
MFC after:	3 weeks
Sponsored by:	Nahanni Systems