Fix handling of Xen(ner) detection

Fix handling of Xen(ner) detection

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