qemu: Wire up <memory/> live update

qemu: Wire up <memory/> live update

As advertised in one of previous commits, we want to be able to
change 'requested-size' attribute of virtio-mem on the fly. This
commit does exactly that. Changing anything else is checked for
and forbidden.

Once guest has changed the allocation, QEMU emits an event which
we will use to track the allocation. In the next commit.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>