qemu: Fix -chardev udp if parameters are omitted

qemu: Fix -chardev udp if parameters are omitted

The following XML:

    <serial type='udp'>
      <source mode='connect' service='9999'/>
    </serial>

is accepted by domain_conf.c but maps to the qemu command line:

-chardev udp,host=127.0.0.1,port=2222,localaddr=(null),localport=(null)

qemu can cope with everything omitting except the connection port, which
seems to also be the intent of domain_conf validation, so let's not
generate bogus command lines for that case.
The defaults are empty strings for addresses and 0 for the localport

Additionally, tweak the qemu cli parsing to handle omitted host
parameters
for -serial udp