qemu: make block io tuning smarter

qemu: make block io tuning smarter

When blkdeviotune was first committed in 0.9.8, we had the limitation
that setting one value reset all others.  But bytes and iops should
be relatively independent.  Furthermore, setting tuning values on
a live domain followed by dumpxml did not output the new settings.

* src/qemu/qemu_driver.c (qemuDiskPathToAlias): Add parameter, and
update callers.
(qemuDomainSetBlockIoTune): Don't lose previous unrelated
settings.  Make live changes reflect to dumpxml output.
* tools/virsh.pod (blkdeviotune): Update documentation.