qemuDomainSetMemoryFlags() would allow to set the initial memory greater
than the <maxMemory> field. While the configuration would not work as
memory hotplug requires NUMA to be enabled and the
qemuDomainSetMemoryFlags() API does not work on NUMA guests this just
fixes a corner case.

The fix is still worth though as it allows to induce an invalid
configuration and make the VM vanish on libvirt restart.

Additionally this tweaks error message to be more accurate.
Signed-off-by: NLuyao Huang <lhuang@redhat.com>
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>