virsh: Update only changed scheduler tunables

virsh: Update only changed scheduler tunables

When setting the cpu tunables in virsh you are able to update only a
subset of them. Virsh while doing the update updated all of the
tunables, changed ones with new values and unchanged with old ones.
This is unfortunate as it:
a) might overwrite some other change by a race condition (unprobable)
b) fails with range checking as some of the old values saved might be
   out of range

This patch changes the update procedure so that only the changed value
is updated on the host.

This patch also fixes a very unprobable memory leak if the daemon would
return a string tunable parameter, as the typed parameter array was not
cleared.