virtio: use bool for boolean functions

Summary:
these static functions:

• vq_ring_use_indirect

and these public functions:

• virtio_bus_is_modern
• virtio_with_feature
• virtqueue_empty
• virtqueue_full

exclusively return a boolean value.
Change their return values to bool.

Sponsored by: The FreeBSD Foundation

Reviewers: bryanv

Reviewed By: bryanv

Subscribers: cognet, imp

Differential Revision: https://reviews.freebsd.org/D41848