qemu: Record the default NIC model in the domain XML

This patch implements the devices post parse callback and uses it to fill
the default qemu network card model into the XML if none is specified.

Libvirt assumes that the network card model for qemu is the "rtl8139".
Record this in the XML using the new callback to avoid user
confusion.

tests/qemuxml2argvdata/qemuxml2argv-net-eth.args

@@ -1,5 +1,5 @@
 LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test /usr/bin/qemu -S -M \
 pc -m 214 -smp 1 -nographic -monitor unix:/tmp/test-monitor,server,nowait \
 -no-acpi -boot c -usb -hda /dev/HostVG/QEMUGuest1 -net nic,\
-macaddr=00:11:22:33:44:55,vlan=0 -net tap,script=/etc/qemu-ifup,vlan=0 -serial \
-none -parallel none
+macaddr=00:11:22:33:44:55,vlan=0,model=rtl8139 -net tap,script=/etc/qemu-ifup,\
+vlan=0 -serial none -parallel none