Add the monitor type to the domain state XML

Add the monitor type to the domain state XML

There are no functional changes in this patch apart from adding the
monitor type to the state XML.

The patch mostly consists of switching to use virDomainChrDef every
where to describe the monitor.

* src/domain_conf.h: replace monitorpath with monitor_chr

* src/domain_conf.c: handle parsing the monitor type and initializing
  monitor chr

* src/qemu_conf.[ch]: make qemudBuildCommandLine take a
  virDomainChrDefPtr and use that to build the -monitor parameter

* src/qemu_driver.c: split pty specific and common code from
  qemudOpenMonitor, have qemudStartVMDaemon() initialize monitor_chr

* tests/qemuxml2argvtest.c: update for qemudBuildCommandLine() change