Description of problem:
when i do provisioning test for scsi-hd interface with discard=on under UNMAP mode, but it fail to discard the free space after remove the data in guest, we could not see any sector rollback.
Also tried the scsi-block did not hit such issue.

Version-Release number of selected component (if applicable):
host info:
3.10.0-60.el7.x86_64
qemu-kvm-rhev-1.5.3-21.el7.x86_64
seabios-1.7.2.2-4.el7.x86_64
guest info:
3.10.0-60.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.generate a scsi disk with scsi_debug using UNMAP mode.
# modprobe scsi_debug dev_size_mb=1024 lbpu=1
# lsscsi
[0:0:0:0]    disk    ATA      ST3500413AS      HP64  /dev/sda 
[2:0:0:0]    cd/dvd  hp       DVD-RAM GH80N    RF01  /dev/sr0 
[6:0:0:0]    disk    EQLOGIC  100E-00          5.0   /dev/sdb 
[12:0:0:0]   disk    Linux    scsi_debug       0004  /dev/sdc 
# cat /sys/block/sdc/device/scsi_disk/12\:0\:0\:0/provisioning_mode
unmap
# cat /sys/bus/pseudo/drivers/scsi_debug/map
                <----------nothing output
2.boot a guest with /dev/sdc as a ide/ahci data disk, like:
# /usr/libexec/qemu-kvm -M pc -S -cpu SandyBridge -enable-kvm...-drive file=/dev/sdc,if=none,id=drive-data-disk,cache=none,format=raw,aio=native,werror=stop,rerror=stop,discard=on -device virtio-scsi-pci,id=scsi1,bus=pci.0,addr=0x7 -device scsi-hd,drive=drive-data-disk,bus=scsi1.0,id=data-disk

3.Make file system (both xfs and ext4 hit it) to the disk in the guest.
# mkfs.xfs /dev/sdb
meta-data=/dev/sdb               isize=256    agcount=4, agsize=65536 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=0
data     =                       bsize=4096   blocks=262144, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=12800, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

4.On the host.
# cat /sys/bus/pseudo/drivers/scsi_debug/map

5.in the guest
# mount /dev/sdb /home/test
# dd if=/dev/zero of=/home/test/file
dd: writing to ‘/home/test/file’: No space left on device
1928857+0 records in
1928856+0 records out
987574272 bytes (988 MB) copied, 1.74507 s, 566 MB/s

6.cat map in host.
# cat /sys/bus/pseudo/drivers/scsi_debug/map

7.in guest.
# rm -f /home/test/file
# fstrim /home/test

8.cat map in host.
# cat /sys/bus/pseudo/drivers/scsi_debug/map

Actual results:
after step 4,
# cat /sys/bus/pseudo/drivers/scsi_debug/map
0-255,524288-524291,524296-524319,1048576-1048579,1048584-1151007,1572864-1572867,1572872-1572895,2096896-2097151
after step 6,
# cat /sys/bus/pseudo/drivers/scsi_debug/map
0-524279,524288-524291,524296-524319,524352-1048567,1048576-1048579,1048584-1151007,1151040-1539975,1572864-1572867,1572872-1572895,1572928-2064447,2096896-2097151
after step 8, the cat map output did not change. 
# cat /sys/bus/pseudo/drivers/scsi_debug/map
0-524279,524288-524291,524296-524319,524352-1048567,1048576-1048579,1048584-1151007,1151040-1539975,1572864-1572867,1572872-1572895,1572928-2064447,2096896-2097151

Expected results:
see SCSI disc map info and compare with step8, we should see some sector rollback, just like scsi-block, like:
e.g:...-drive file=/dev/sdc,if=none,id=drive-data-disk,cache=none,format=raw,aio=native,werror=stop,rerror=stop,discard=on -device virtio-scsi-pci,id=scsi1,bus=pci.0,addr=0x7 -device scsi-block,drive=drive-data-disk,bus=scsi1.0,id=data-disk

after step 4,
# cat /sys/bus/pseudo/drivers/scsi_debug/map
0-255,262144-262147,262152-262175,524288-524291,524296-524319,786432-786435,786440-786463,1048576-1048579,1048584-1151007,1310720-1310723,1310728-1310751,1572864-1572867,1572872-1572895,1835008-1835011,1835016-1835039,2096896-2097151
after step 6,
# cat /sys/bus/pseudo/drivers/scsi_debug/map
0-262135,262144-262147,262152-262175,262208-524279,524288-524291,524296-524319,524352-786423,786432-786435,786440-786463,786496-1048567,1048576-1048579,1048584-1151007,1151040-1310479,1310720-1310723,1310728-1310751,1310784-1572855,1572864-1572867,1572872-1572895,1572928-1769607,1769616-1834991,1835008-1835011,1835016-1835039,1835072-2031839,2096896-2097151
after step 8,
# cat /sys/bus/pseudo/drivers/scsi_debug/map
0-95,262144-262147,262152-262175,524288-524291,524296-524319,786432-786435,786440-786463,1048576-1048579,1048584-1151007,1310720-1310723,1310728-1310751,1572864-1572867,1572872-1572895,1835008-1835011,1835016-1835039

Additional info: