Now that the size of guest's memory can be inferred from the NUMA
configuration (if present) make it optional to specify <memory>
explicitly.

To make sure that memory is specified add a check that some form of
memory size was specified. One side effect of this change is that it is
no longer possible to specify 0KiB as memory size for the VM, but I
don't think it would be any useful to do so. (I can imagine embedded
systems without memory, just registers, but that's far from what libvirt
is usually doing).

Forbidding 0 memory for guests also fixes a few corner cases where 0 was
not interpreted correctly and caused failures. (Arguments for numad when
using automatic placement, size of the balloon). This fixes problems
described in https://bugzilla.redhat.com/show_bug.cgi?id=1161461

Test case changes are added to verify that the schema change and code
behave correctly.