Add VIRTIO_BLK_T_DISCARD support to the virtio-blk driver
If the hypervisor advertises support for the DISCARD command then the
guest can perform TRIM commands, freeing space on the backing store.
If VIRTIO_BLK_F_DISCARD is enabled, advertise DISKFLAG_CANDELETE
Tested with FreeBSD guests on bhyve and KVM
Reviewed by: jhb
Tested by: freqlabs
MFC after: 1 month
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D21708