Domiftune changes

There are two commits within this change - the first just changes the value of the maximum value allowed. Libvirt expects a kilobyte value not a byte value for input. Also there was a rounding bug in the package libvirt calls so the issue was masked; however, recent versions of that code fix the rounding problem - thus this showed as a error in the test.  Note that the libvirt documentation was updated to more clearly reflect the kilobyte value.  The second commit handles an upstream change to allow using the value "0" (zero) as a mechanism to clear the current value - thus this code will handle that.  It requires PR #82 to be pushed first however.