qemu: Make Set*Mem commands hotplug only

qemu: Make Set*Mem commands hotplug only

SetMem and SetMaxMem are hotplug only APIs, any persistent config
changes are supposed to go via XML definition. The original implementation
of these calls were incorrect and had the nasty side effect of making
a psuedo persistent change that would be lost after libvirtd restart
(I didn't know any better).

Fix these APIs to rightly reject non running domains.

Signed-off-by: Cole Robinson <crobinso@redhat.com>