- Add bhyve virtio-scsi storage backend support.

- Add bhyve virtio-scsi storage backend support.

Example of configuration:
ctl.conf:
portal-group pg0 {
        discovery-auth-group no-authentication
        listen 0.0.0.0
        listen [::]
}

target iqn.2012-06.com.example:target0 {
        auth-group no-authentication
        portal-group pg0
        port ioctl/5/3

        lun 0 {
                path /z/test.img
                size 8G
        }
        lun 1 {
                path /z/test1.img
                size 8G
        }
}

bhyve <...> -s 4,virtio-scsi,/dev/cam/ctl5.3,iid=3 <VM_NAME>

From inside guest:
root@:~ # zpool status test
  pool: test
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        test        ONLINE       0     0     0
          da0       ONLINE       0     0     0
          da1       ONLINE       0     0     0

dmesg:
da0 at vtscsi0 bus 0 scbus0 target 0 lun 0
da0: <FREEBSD CTLDISK 0001> Fixed Direct Access SPC-5 SCSI device
da0: Serial Number MYSERIAL0000
da0: 300.000MB/s transfers
da0: Command Queueing enabled
da0: 8192MB (16777216 512 byte sectors)
da1 at vtscsi0 bus 0 scbus0 target 0 lun 1
da1: <FREEBSD CTLDISK 0001> Fixed Direct Access SPC-5 SCSI device
da1: Serial Number MYSERIAL0001
da1: 300.000MB/s transfers
da1: Command Queueing enabled
da1: 8192MB (16777216 512 byte sectors)

Discussed with:		grehan
Reviewed by:		mav
Obtained from:		TrueOS
Relnotes:		Yes
Sponsored by:		iXsystems Inc.
Tested with:		FreeBSD HEAD, Fedora 28 (Workstation) and
			Ubuntu 18.04.
Differential Revision:  https://reviews.freebsd.org/D15276