Incorrect warnings when using vhost without numa
Description of problem:
Part A: Misleading error message. Running the above command for any architecture fails to initialize vhost, and prints the following, incorrect advice
```
qemu-system-mips: Failed initializing vhost-user memory map, consider using -object memory-backend-file share=on
qemu-system-mips: vhost_set_mem_table failed: Invalid argument (22)
qemu-system-mips: Error starting vhost: 22
```

Since the command line already contains `-object memory-backend-file,id=mem1,mem-path=/tmp/mem,size=256M,share=on` this error message should not be printed. For x86_64, this can be resolved by adding `-numa node,memdev=mem0` to the command line. As such, I think this error message should instead guide a user to adding that argument.

Part B: No documented configuration to run vhost-user for machines that don't support numa.
The mips malta machine does not support the `-numa` flag. It is unclear if this means that `vhost` cannot be used with this platform or if a non-numa configuration with a memory-backend-file can be used.
Steps to reproduce:
1. Run `vhost-user-vsock --socket=/tmp/vhost4.socket --uds-path=/tmp/foo` from https://github.com/rust-vmm/vhost-device.
1. Run the above QEMU command