* src/qemu/qemu_conf.c: add the ",readonly=on" for read-only disks
  and also parse it back in qemuParseCommandLineDisk()
* tests/qemuxml2argvtest.c
  tests/qemuxml2argvdata/qemuxml2argv-disk-drive-readonly-disk.args
  tests/qemuxml2argvdata/qemuxml2argv-disk-drive-readonly-disk.xml:
  add a specific regression test