numa: prevent usage of -M memory-backend and -numa memdev at the same time

numa: prevent usage of -M memory-backend and -numa memdev at the same time

Options -M memory-backend and -numa memdev are mutually exclusive,
and if used together, it might lead to a crash in the worst case.
For example when the same backend is used with these options together:
  -m 4G \
  -object memory-backend-ram,id=mem0,size=4G \
  -M pc,memory-backend=mem0 \
  -numa node,memdev=mem0
QEMU will abort with:
   exec.c:2006: qemu_ram_set_idstr: Assertion `!new_block->idstr[0]' failed.

and following backtrace:
    abort ()
    qemu_ram_set_idstr ()
    vmstate_register_ram ()
    vmstate_register_ram_global ()
    machine_consume_memdev ()
    numa_init_memdev_container ()
    numa_complete_configuration ()
    machine_run_board_init ()

add a check to error out in case the user tries to use both options at
the same time.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200511141103.43768-3-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>