disk: Fix virt-xml --add-device --disk size=X

tests/cli-test-xml/compare/virt-xml-add-disk-default-storage.xml

@@ -0,0 +1,13 @@
+     <panic>
+       <address type="isa" iobase="0x505"/>
+     </panic>
++    <disk type="file" device="disk">
++      <source file="/var/lib/libvirt/images/test-for-virtxml.qcow2"/>
++      <target dev="hdb" bus="ide"/>
++    </disk>
+   </devices>
+   <seclabel type="dynamic" model="selinux" relabel="yes"/>
+ </domain>
+
+Domain 'test-for-virtxml' defined successfully.
+Changes will take effect after the domain is fully powered off.

tests/clitest.py

@@ -967,6 +967,7 @@ c.add_compare("--add-device --sound pcspk", "add-sound")
 c.add_compare("--add-device --disk %(EXISTIMG1)s,bus=virtio,target=vdf", "add-disk-basic")
 c.add_compare("--add-device --disk %(EXISTIMG1)s", "add-disk-notarget")  # filling in acceptable target
 c.add_compare("--add-device --disk %(NEWIMG1)s,size=.01", "add-disk-create-storage")
+c.add_compare("--add-device --disk size=.01", "add-disk-default-storage")
 c.add_compare("--remove-device --sound ich6", "remove-sound-model", check_version="1.3.5")  # check_version=new graphics listen output
 c.add_compare("--remove-device --disk 3", "remove-disk-index")
 c.add_compare("--remove-device --disk /dev/null", "remove-disk-path")

virtinst/devices/disk.py

@@ -413,6 +413,8 @@ class DeviceDisk(Device):
     def get_vol_object(self):
         return self._storage_backend.get_vol_object()
     def get_vol_install(self):
+        if not self._storage_backend:
+            return None
         return self._storage_backend.get_vol_install()
     def get_parent_pool(self):
         if self.get_vol_install():