allows the following to be specified in a domain:
<channel type='pipe'>
  <source path='/tmp/guestfwd'/>
  <target type='guestfwd' address='10.0.2.1' port='4600'/>
</channel>

* proxy/Makefile.am: add network.c as dep of domain_conf.c
* docs/schemas/domain.rng src/conf/domain_conf.[ch]: extend the domain
  schemas and the parsing/serialization side for the new construct

QEmu support will add the following on the qemu command line:
 -chardev pipe,id=channel0,path=/tmp/guestfwd
 -net user,guestfwd=tcp:10.0.2.1:4600-chardev:channel0

* src/qemu/qemu_conf.c: Add argument output for channel
* tests/qemuxml2(argv|xml)test.c: Add test for <channel> domain syntax