* src/conf/domain_conf.h: Add new member "target" to struct
_virDomainDeviceDriveAddress.
* src/conf/domain_conf.c: Parse and format "target"
* Lots of tests (.xml) in tests/domainsnapshotxml2xmlout,
tests/qemuxml2argvdata, tests/qemuxml2xmloutdata, and
tests/vmx2xmldata/ are modified for newly introduced
attribute "target" for address of "drive" type.