virDomainDiskDefAssignAddress: return int, not void

virDomainDiskDefAssignAddress: return int, not void

Before, this function would blindly accept an invalid def->dst
and then abuse the idx=-1 it would get from virDiskNameToIndex,
when passing it invalid strings like "xvda:disk" and "sda1".
Now, this function returns -1 upon failure.
* src/conf/domain_conf.c (virDomainDiskDefAssignAddress): as above.
Update callers.
* src/conf/domain_conf.h: Update prototype.
* src/qemu/qemu_conf.c: Update callers.