We need to store things like device names and PCI slot numbers in the
qemu domain state file so that we don't lose that information on
libvirtd restart. Add a flag to indicate that this information should
be parsed or formatted.

Make bit 16 and above of the flags bitmask for internal use only and
consume the first bit for this new status flag.

* include/libvirt/libvirt.h: add VIR_DOMAIN_XML_FLAGS_MASK

* src/libvirt.c: reject private flags in virDomainGetXMLDesc()

* src/domain_conf.h: add VIR_DOMAIN_XML_INTERNAL_STATUS

* src/domain_conf.c: pass the flag from virDomainObjParseXML() and
  virDomainSaveStatus