Do xml->xml test for usb-redir-filter

We don't format the default '-1' fields back.

tests/qemuxml2xmloutdata/qemuxml2xmlout-usb-redir-filter.xml

+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219136</memory>
+  <currentMemory unit='KiB'>219136</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <controller type='usb' index='0' model='ich9-ehci1'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x7'/>
+    </controller>
+    <controller type='usb' index='0' model='ich9-uhci1'>
+      <master startport='0'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0' multifunction='on'/>
+    </controller>
+    <controller type='usb' index='0' model='ich9-uhci2'>
+      <master startport='2'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x1'/>
+    </controller>
+    <controller type='usb' index='0' model='ich9-uhci3'>
+      <master startport='4'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x2'/>
+    </controller>
+    <controller type='pci' index='0' model='pci-root'/>
+    <redirdev bus='usb' type='spicevmc'>
+      <address type='usb' bus='0' port='4'/>
+    </redirdev>
+    <redirdev bus='usb' type='spicevmc'>
+      <address type='usb' bus='0' port='5'/>
+    </redirdev>
+    <redirfilter>
+      <usbdev class='0x08' vendor='0x15E1' product='0x2007' version='1.10' allow='yes'/>
+      <usbdev allow='no'/>
+    </redirfilter>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>

tests/qemuxml2xmltest.c

@@ -504,6 +504,7 @@ mymain(void)
     DO_TEST("virtio-lun");
 
     DO_TEST("usb-redir");
+    DO_TEST_DIFFERENT("usb-redir-filter");
     DO_TEST("blkdeviotune");
 
     DO_TEST_FULL("seclabel-dynamic-baselabel", false, WHEN_INACTIVE);