qemu: Add NUMA node automatically for memory hotplug

qemu: Add NUMA node automatically for memory hotplug

Up until v2.11.0-rc2~19^2~3 QEMU used to require at least one
NUMA node to be configured when memory hotplug was enabled. After
that commit, QEMU automatically adds a NUMA node if none was
specified on the cmd line. Reflect this in domain XML, i.e.
explicitly add a NUMA node into our domain definition if needed.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2216236
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>