qemu: Improve error when setting invalid count of vcpus via agent

qemu: Improve error when setting invalid count of vcpus via agent

https://bugzilla.redhat.com/show_bug.cgi?id=1035108

When attempting to enable more vCPUs in the guest than is currently
enabled in the guest but less than the maximum count for the VM we
currently reported an unhelpful message:

 error: internal error: guest agent reports less cpu than requested

This patch changes it to:

 error: invalid argument: requested vcpu count is greater than the count
 of enabled vcpus in the domain: 3 > 2