Introduce a new structure

      struct _virDomainDeviceDriveAddress {
        unsigned int controller;
        unsigned int bus;
        unsigned int unit;
      };

and plug that into virDomainDeviceAddress and generates XML that
looks like

  <address type='drive' controller='1' bus='0' unit='5'/>

This syntax will be used by the QEMU driver to explicitly control
how drives are attached to the bus

* src/conf/domain_conf.h, src/conf/domain_conf.c: Parsing and
  formatting of drive addresses
* docs/schemas/domain.rng: Define new address format for drives