cli: Fix unsetting --disk path

tests/cli-test-xml/compare/virtxml-edit-simple-disk-remove-path.xml

@@ -0,0 +1,13 @@
+--- Original XML
++++ Altered XML
+@@ -55,7 +55,6 @@
+   <devices>
+     <emulator>/usr/lib/xen/bin/qemu-dm</emulator>
+     <disk type="block" device="floppy">
+-      <source dev="/dev/null"/>
+       <target dev="fda" bus="fdc"/>
+       <address type="drive" controller="0" bus="0" target="0" unit="0"/>
+     </disk>
+
+Domain 'test-many-devices' defined successfully.
+Changes will take effect after the next domain shutdown.

tests/clitest.py

@@ -787,6 +787,7 @@ c.add_compare("--security label=foo,bar,baz,UNKNOWN=val,relabel=on", "virtxml-ed
 c.add_compare("--features eoi=on,hyperv_relaxed=off,acpi=", "virtxml-edit-simple-features")
 c.add_compare("--clock offset=localtime,hpet_present=yes,kvmclock_present=no,rtc_tickpolicy=merge", "virtxml-edit-simple-clock")
 c.add_compare("--disk /dev/zero,perms=ro,startup_policy=optional", "virtxml-edit-simple-disk")
+c.add_compare("--disk path=", "virtxml-edit-simple-disk-remove-path")
 c.add_compare("--network source=br0,type=bridge,model=virtio,mac=", "virtxml-edit-simple-network")
 c.add_compare("--graphics tlsport=5902,keymap=ja", "virtxml-edit-simple-graphics")
 c.add_compare("--controller index=2,model=lsilogic", "virtxml-edit-simple-controller")

virtinst/cli.py

@@ -1723,6 +1723,7 @@ class ParserDisk(VirtCLIParser):
         convert_perms(opts.get_opt_param("perms"))
 
         path = opts.get_opt_param("path")
+        had_path = path is not None
         backing_store = opts.get_opt_param("backing_store")
         pool = opts.get_opt_param("pool")
         vol = opts.get_opt_param("vol")
@@ -1734,8 +1735,8 @@ class ParserDisk(VirtCLIParser):
             self.guest, path, pool, vol, size, fmt, sparse)
 
         path = volobj and volobj.path() or abspath
-        if path:
-            opts.opts["path"] = path
+        if had_path or path:
+            opts.opts["path"] = path or ""
 
         inst = VirtCLIParser._parse(self, opts, inst)