qemu: Remove generation of drive alias from qcow passphrase backends

qemu: Remove generation of drive alias from qcow passphrase backends

Rather than pass the disks[i]->info.alias to qemuMonitorSetDrivePassphrase
and then generate the "drive-%s" alias from that, let's use qemuAliasFromDisk
prior to the call to generate the drive alias and then pass that along
thus removing the need to generate the alias from the monitor code.