libxl/vcpu-set - allow to decrease vcpu count on overcommitted guests (v5)

libxl/vcpu-set - allow to decrease vcpu count on overcommitted guests (v5)

We have a check to warn the user if they are overcommitting.
But the check only checks the hosts CPU amount and does
not take into account the case when the user is trying to fix
the overcommit. That is - they want to limit the amount of
online VCPUs.

This fix allows the user to offline vCPUs without any
warnings when they are running an overcommitted guest.

Also fix the extra space in the message.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>