numa: allow memory-less nodes when using memdev as backend

numa: allow memory-less nodes when using memdev as backend

QEMU fails to start if memory-less node is present when memdev
is used
  qemu-system-x86_64 -object memory-backend-ram,id=ram0,size=128M \
                     -numa node -numa node,memdev=ram0
with error:
  "memdev option must be specified for either all or no nodes"

which works as expected if legacy 'mem' is used.

Fix check to make memory-less nodes valid when memdev option is used
but still disallow mix of mem and memdev options.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20190702140745.27767-2-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>