virtinst: Support transient.shareBacking option

Implement "<transient shareBacking=yes/>" to virtinst to allow a transient disk
to be shared across VMs. It is introduced to libvirt since:

75871da0ec    qemu: Allow <transient> disks with images shared accross
VMs

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Han Han <hhan@redhat.com>

tests/data/cli/compare/virt-install-many-devices.xml

@@ -216,6 +216,7 @@
       <driver name="qemu" type="raw"/>
       <source file="/var/lib/libvirt/images/disk.img"/>
       <target dev="vdp" bus="virtio"/>
+      <transient shareBacking="yes"/>
     </disk>
     <disk type="block" device="disk">
       <driver name="qemu" type="raw" cache="none" io="native"/>

tests/test_cli.py

@@ -608,7 +608,7 @@ vcpus.vcpu1.id=2,vcpus.vcpu1.enabled=yes
 --disk path=/dev/disk-pool/diskvol7,device=lun,bus=scsi,reservations.managed=no,reservations.source.type=unix,reservations.source.path=/var/run/test/pr-helper0.sock,reservations.source.mode=client,\
 source.reservations.managed=no,source.reservations.source.type=unix,source.reservations.source.path=/var/run/test/pr-helper0.sock,source.reservations.source.mode=client,target.rotation_rate=6000
 --disk vol=iscsi-direct/unit:0:0:1
---disk size=.0001,format=raw
+--disk size=.0001,format=raw,transient=on,transient.shareBacking=yes
 --disk size=.0001,pool=disk-pool
 --disk path=%(EXISTIMG1)s,type=dir
 --disk path=/fooroot.img,size=.0001,transient=on

virtinst/cli.py

@@ -3441,6 +3441,7 @@ class ParserDisk(VirtCLIParser):
         cls.add_arg("readonly", "read_only", is_onoff=True)
         cls.add_arg("shareable", "shareable", is_onoff=True)
         cls.add_arg("transient", "transient", is_onoff=True)
+        cls.add_arg("transient.shareBacking", "transient_shareBacking", is_onoff=True)
 
         cls.add_arg("target.bus", "bus")
         cls.add_arg("target.removable", "removable", is_onoff=True)

virtinst/devices/disk.py

@@ -476,6 +476,7 @@ class DeviceDisk(Device):
     read_only = XMLProperty("./readonly", is_bool=True)
     shareable = XMLProperty("./shareable", is_bool=True)
     transient = XMLProperty("./transient", is_bool=True)
+    transient_shareBacking = XMLProperty("./transient/@shareBacking", is_yesno=True)
     driver_cache = XMLProperty("./driver/@cache")
     driver_discard = XMLProperty("./driver/@discard")
     driver_detect_zeroes = XMLProperty("./driver/@detect_zeroes")