numa: check for busy memory backend

Specifying the same memory backend twice leads to an assert:

./x86_64-softmmu/qemu-system-x86_64 -m 512M -enable-kvm -object
memory-backend-ram,size=256M,id=ram0 -numa node,nodeid=0,memdev=ram0
-numa node,nodeid=1,memdev=ram0
qemu-system-x86_64: /scm/qemu/memory.c:1506:
memory_region_add_subregion_common: Assertion `!subregion->container'
failed.
Aborted (core dumped)

Detect and exit with an error message instead.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
