Currently, monitoring QEMU virtual machines with standard Unix
sysadmin tools is harder than it has to be. The QEMU command line is
often miles long and mostly redundant, it's hard to tell which process
is which.

This patch reorders the QEMU -name argument to be the first, so it's
immediately visible in "ps x", htop and "atop -c" output.

This patch teaches testutil how to read multi-line input files with
backspace-newline line continuation markers.

The patch also breaks up all the single-line arguments test input files into
multi-line files with lines shorter than 80 characters.

Latest upstream QEMU can be built with Xen support, which introduces
a -xen-domid argument. This was  mistakenly detected as -domid due
to old Xenner support. Adapt to cope with both syntax. Also only
set domid if the virt type is xen, or the guest type is xen

* src/qemu_conf.c, src/qemu_conf.h: Detect new -xen-domid flag in
  preference to -domid.
* tests/qemuxml2argvdata/qemuxml2argv-bootloader.args,
  tests/qemuxml2argvdata/qemuxml2argv-input-xen.args: Add missing
  -domid param
* tests/qemuxml2argvdata/qemuxml2argv-misc-uuid.args: Remove bogus
  -boot param.
* tests/qemuxml2argvtest.c: Add missing QEMUD_CMD_FLAG_DOMID params

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.