tests: Test XML entities in source path for RNG device

Users may want to specify XML entities in paths to devices. Ensure they
are parsed and used properly.

tests/qemuxml2argvdata/qemuxml2argv-virtio-rng-random.args

-LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test /usr/bin/qemu -S -M pc -m 214 -smp 1 -nographic -nodefaults -monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -usb -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3 -object rng-random,id=rng0,filename=/test/phile -device virtio-rng-pci,rng=rng0,bus=pci.0,addr=0x4
+LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test /usr/bin/qemu \
+-S -M pc -m 214 -smp 1 -nographic -nodefaults \
+-monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -usb \
+-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3 \
+-object 'rng-random,id=rng0,filename=/test/ph<ile' \
+-device virtio-rng-pci,rng=rng0,bus=pci.0,addr=0x4

tests/qemuxml2argvdata/qemuxml2argv-virtio-rng-random.xml

@@ -17,7 +17,7 @@
     <controller type='usb' index='0'/>
     <memballoon model='virtio'/>
     <rng model='virtio'>
-      <backend model='random'>/test/phile</backend>
+      <backend model='random'>/test/ph&lt;ile</backend>
     </rng>
   </devices>
 </domain>