Change done by commit f309db1f wrongly
assumes that qemu can start with a combination of NUMA nodes specified
with the "memdev" option and the appropriate backends, and the legacy
way by specifying only "mem" as a size argument. QEMU rejects such
commandline though:

$ /usr/bin/qemu-system-x86_64 -S -M pc -m 1024 -smp 2 \
-numa node,nodeid=0,cpus=0,mem=256 \
-object memory-backend-ram,id=ram-node1,size=12345 \
-numa node,nodeid=1,cpus=1,memdev=ram-node1
qemu-system-x86_64: -numa node,nodeid=1,cpus=1,memdev=ram-node1: qemu: memdev option must be specified for either all or no nodes

To fix this issue we need to check if any of the nodes requires the new
definition with the backend and if so, then all other nodes have to use
it too.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1182467