virsh: improve memory unit parsing

virsh: improve memory unit parsing

The last vestige of the inaccurate 'kilobytes' when we meant 1024 is
now gone.  And virsh is now useful for setting memory in units other
than KiB.

* tools/virsh.c (cmdSetmem, cmdSetmaxmem): Use new helper routine,
allow passing bogus arguments on to hypervisor to test driver
sanity checking, and fix leak on parse error.
(vshMemtuneGetSize): New helper.
(cmdMemtune): Use it.
* tools/virsh.pod (setmem, setmaxmem, memtune): Document this.