qemu: parse -device virtio-balloon

There are no options to parse here other than the name of the device,
and all three possible device names have the same prefix
("virtio-balloon" with "-ccw", "-pci", or "-device" appended), so the
code is fairly simple. It has been implemented such that it will be
easier to add handling for other -device entries that aren't otherwise
recognized - just add another "else if (STRPREFIX(opts, ....)" clause.

qemuParseCommandLineString() previously would always add a <memballoon
model='virtio'/> to every result (the comments erroneously say that it
is adding a <memballoon model='none'/>) This has been changed to add
model='none', and 84 test case xml's updated accordingly (so that
qemuxml2argvtest won't fail).

Now that the memballoon device is properly parsed, we can safely add a
test for properly ignoring -nodefconfig and -nodefaults. Rather than
adding an entire new test case for this (and memballoon), we just
randomly pick the clock-utc test and modify it slightly to fulfill the
purpose.

tests/qemuxml2argvdata/qemuxml2argv-serial-tcp.xml

@@ -33,6 +33,6 @@
       <protocol type='raw'/>
       <target type='serial' port='0'/>
     </console>
-    <memballoon model='virtio'/>
+    <memballoon model='none'/>
   </devices>
 </domain>