* docs/schemas/domain.rng: Add <serial> element to disks
* src/domain_conf.h, src/domain_conf.c: XML parsing and
  formatting for disk serial numbers
* src/qemu_conf.c: Set serial number when launching guests
* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-shared.args,
  tests/qemuxml2argvdata/qemuxml2argv-disk-drive-shared.xml: Add
  serial number to XML test