diskbackend: Simplify path a case of path joining

We are reimplementing os.path.join. Make sure the standard cli
test suite hits this case too

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

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

@@ -227,6 +227,11 @@
       <source dir="/dev/default-pool/testvol1.img"/>
       <target dev="vdr" bus="virtio"/>
     </disk>
+    <disk type="file" device="disk">
+      <driver name="qemu" type="qcow2"/>
+      <source file="/fooroot.img"/>
+      <target dev="vds" bus="virtio"/>
+    </disk>
     <controller type="usb" index="0" model="ich9-ehci1">
       <address type="pci" domain="0" bus="0" slot="4" function="7"/>
     </controller>

tests/test_cli.py

@@ -612,6 +612,7 @@ source.reservations.managed=no,source.reservations.source.type=unix,source.reser
 --disk size=.0001,format=raw
 --disk size=.0001,pool=disk-pool
 --disk path=%(EXISTIMG1)s,type=dir
+--disk path=/fooroot.img,size=.0001
 
 --network user,mac=12:34:56:78:11:22,portgroup=foo,link_state=down,rom_bar=on,rom_file=/tmp/foo
 --network bridge=foobar,model=virtio,driver_name=qemu,driver_queues=3,filterref=foobar,rom.bar=off,rom.file=/some/rom,source.portgroup=foo

virtinst/diskbackend.py

@@ -436,11 +436,8 @@ class _StorageCreator(_StorageBase):
             if self.get_dev_type() == "network":
                 self._path = self._vol_install.name
             else:
-                sep = "/"
-                if xmlobj.target_path == "" or xmlobj.target_path[-1] == '/':
-                    sep = ""
-                self._path = (xmlobj.target_path + sep +
-                              self._vol_install.name)
+                self._path = os.path.join(
+                        xmlobj.target_path, self._vol_install.name)
         return self._path
 
     def get_vol_install(self):