Get QEMU pty paths from the monitor

Get QEMU pty paths from the monitor

This change makes the QEMU driver get pty paths from the output of the
monitor 'info chardev' command. This output is structured, and contains
both the name of the device and the path on the same line. This is
considerably more reliable than parsing the startup log output, which
requires the parsing code to know which order QEMU will print pty
information in.

Note that we still need to parse the log output as the monitor itself
may be on a pty. This should be rare, however, and the new code will
replace all pty paths parsed by the log output method once the monitor
is available.

* src/qemu/qemu_monitor.(c|h) src/qemu_monitor_text.(c|h): Implement
  qemuMonitorGetPtyPaths().
* src/qemu/qemu_driver.c: Get pty path information using
  qemuMonitorGetPtyPaths().