Move legacy interrupt allocation for virtio devices to common code.
There are a number of assumptions about legacy interrupts always
being available in virtio so don't allow back-ends to make the
decision to support them.

This fixes the issue seen with virtio-rnd on OpenBSD. MSI-x vectors
were not being used, and the virtio-rnd backend wasn't allocating a
legacy interrupt resulting in a bhyve assert and guest exit.

Reported by: Julian Hsiao, madoka at nyanisore dot net
Reviewed by: neel
MFC after: 1 week