tests: clitest: Add explicit coverage for virt-xml unsetting

For example, '--edit --disk path=' can be used to unset a disk path.
Internally we map empty string to None

tests/cli-test-xml/compare/virt-xml-edit-disk-unset.xml

@@ -0,0 +1,14 @@
+   <devices>
+     <emulator>/usr/lib/xen/bin/qemu-dm</emulator>
+     <disk type="block" device="floppy">
+-      <source dev="/dev/null"/>
+-      <target dev="fda" bus="fdc"/>
+-      <boot order="5"/>
++      <target dev="fdb" bus="fdc"/>
++      <boot order="12"/>
+       <address type="drive" controller="0" bus="0" target="0" unit="0"/>
+     </disk>
+     <disk type="file" device="disk">
+
+Domain 'test-for-virtxml' defined successfully.
+Changes will take effect after the domain is fully powered off.

tests/clitest.py

@@ -990,6 +990,7 @@ c.add_compare("--edit target=hda --disk /dev/null", "edit-select-disk-target")
 c.add_compare("--edit /tmp/foobar2 --disk shareable=off,readonly=on", "edit-select-disk-path")
 c.add_compare("--edit mac=00:11:7f:33:44:55 --network target=nic55", "edit-select-network-mac")
 c.add_compare("--edit target=hda --disk boot_order=1", "edit-select-disk-bootorder")
+c.add_compare("--edit path=/dev/null --disk path=,target=fdb,boot_order=12", "edit-disk-unset")  # --disk matching, using empty value to unset path
 
 c = vixml.add_category("edit and start selection", "test-state-shutoff --print-diff --start")
 c.add_compare("--define --edit target=vda --disk boot_order=1", "start-select-disk-bootorder")