* qemuxml2argvdata/qemuxml2argv-graphics-vnc-sasl.args,
  qemuxml2argvdata/qemuxml2argv-graphics-vnc-tls.args,
  qemuxml2argvdata/qemuxml2argv-graphics-vnc.args,
  qemuxml2argvdata/qemuxml2argv-input-xen.args: Add in
  QEMU_AUDIO_DRV=none env variable

We keep support for the pty based monitor so that we can re-connect
to VMs started by older versions of libvirtd.

* src/domain_conf.c: handle formatting and parsing unix monitors

* src/qemu_driver.c: add qemudOpenMonitorUnix(), remove the monitor
  pty path searching from qemudFindCharDevicePTYs(), switch
  qemudStartVMDaemon() and qemuDomainXMLToNative() to using a unix
  monitor

* tests/qemuxml2argvtest.c: switch to using a unix monitor

* tests/qemuxml2argvdata/qemuxml2argv-*.args: update test data

virExec will write out the pid of the daemonized process only. Use this
in the QEMU driver, rather than QEMU's pidfile, so we can catch errors we
might miss if the emulator bails early.