Move the logic from virDomainDiskDefDstDuplicates into
virDomainDiskDefCheckDuplicateInfo so that we don't have to loop
multiple times through the array of disks. Since the original function
was called in qemuBuildDriveDevStr, it was actually called for every
single disk which was quite wasteful.

Additionally the target uniqueness check needed to be duplicated in
the disk hotplug case, since the disk was inserted into the domain
definition after the device string was formatted and thus
virDomainDiskDefDstDuplicates didn't do anything in that case.