Disable memory hotplugging for custom NUMA topologies

Disable memory hotplugging for custom NUMA topologies

This cannot work, since we adjust the 'memory' property of the VM config
on hotplugging, but then the user-defined NUMA topology won't match for
the next start attempt.

Check needs to happen here, since it otherwise fails early with "total
memory for NUMA nodes must be equal to vm static memory".

With this change the error message reflects what is actually happening
and doesn't allow VMs with exactly 1GB of RAM either.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Tested-by: Alwin Antreich <a.antreich@proxmox.com>