virtinst: disk: Fix seclabel testing

disk devices don't have a full <seclabel> under their <source>
element, just a smaller one with only a few options. Fix things
to match what libvirt does

tests/xmlparse-xml/change-disk-in.xml

@@ -26,7 +26,7 @@
     <disk type="block" device="disk">
       <source dev="/dev/null">
         <seclabel model='selinux' relabel='no'/>
-        <seclabel model='dac' type='dynamic'/>
+        <seclabel model='dac'/>
       </source>
       <target dev="hdb" bus="ide"/>
     </disk>

tests/xmlparse-xml/change-disk-out.xml

@@ -27,7 +27,9 @@
     <disk type="block" device="disk">
       <source dev="/dev/null">
         <seclabel model="selinux" relabel="no"/>
-        <seclabel model="dac" type="none"/>
+        <seclabel model="dac" relabel="yes">
+          <label>foo-my-label</label>
+        </seclabel>
       </source>
       <target dev="hdb" bus="ide"/>
     </disk>

tests/xmlparse.py

@@ -352,7 +352,8 @@ class XMLParseTest(unittest.TestCase):
         disk = guest.get_devices("disk")[1]
         check = self._make_checker(disk.seclabel[1])
         check("model", "dac")
-        check("type", "dynamic", "none")
+        check("relabel", None, True)
+        check("label", None, "foo-my-label")
 
         disk = _get_disk("hdc")
         check = self._make_checker(disk)

virtinst/devicedisk.py

@@ -30,7 +30,7 @@ from . import diskbackend
 from . import util
 from .device import VirtualDevice
 from .seclabel import Seclabel
-from .xmlbuilder import XMLChildProperty, XMLProperty
+from .xmlbuilder import XMLBuilder, XMLChildProperty, XMLProperty
 
 
 def _qemu_sanitize_drvtype(phystype, fmt, manual_format=False):
@@ -91,6 +91,17 @@ def _is_dir_searchable(uid, username, path):
     return bool(re.search("user:%s:..x" % username, out))
 
 
+class _DiskSeclabel(XMLBuilder):
+    """
+    This is for disk source <seclabel>. It's similar to a domain
+    <seclabel> but has fewer options
+    """
+    _XML_ROOT_NAME = "seclabel"
+    model = XMLProperty("./@model")
+    relabel = XMLProperty("./@relabel", is_yesno=True)
+    label = XMLProperty("./label")
+
+
 class VirtualDisk(VirtualDevice):
     virtual_device_type = VirtualDevice.VIRTUAL_DEV_DISK
 
@@ -742,7 +753,7 @@ class VirtualDisk(VirtualDevice):
     iotune_wbs = XMLProperty("./iotune/write_bytes_sec", is_int=True)
     iotune_wis = XMLProperty("./iotune/write_iops_sec", is_int=True)
 
-    seclabel = XMLChildProperty(Seclabel, relative_xpath="./source")
+    seclabel = XMLChildProperty(_DiskSeclabel, relative_xpath="./source")
 
 
     #################################