qemu_devices: Fix network creation problem with RHEL5

We were assembling the network device cmd line for RHEL5
QEMU incorrectly, post the qemu_devices work.

 bad cmdline:
     -net id=idprh29y,macaddr=9a:49:4a:4b:4c:4d,model=virtio,type=nic,vlan=0  \
     -net tap,vlan=0,fd=24  \

 good cmdline:
     -net nic,vlan=0,model=virtio,macaddr='9a:21:22:23:24:25',id='idxlzqAk' \
     -net tap,vlan=0,fd=24 \

Fix this by making the 'nic' param of the custom QDevice
NO_STRING_EQUAL, and making sure such params come first before others.

CC: Lukas Doktor <ldoktor@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>