Add VIRTIO_BLK_T_DISCARD (TRIM) support to the bhyve virtio-blk backend

This will advertise support for TRIM to the guest virtio-blk driver and
perform the DIOCGDELETE ioctl on the backing storage if it supports it.

Thanks to Jason King and others at Joyent and illumos for expanding on
my original patch, adding improvements including better error handling
and making sure to following the virtio spec.

Submitted by: Jason King <jason.king@joyent.com> (improvements)
Reviewed by: jhb
Obtained from: illumos-joyent (improvements)
MFC after: 1 month
Relnotes: yes
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D21707