IVSHMEM device doesn't work for sharing memory with virtiofsd
Description of problem:
Trying to share a folder on the host to the guest with `virtiofsd` using the `ivshmem-plain` device doesn't work (for memory sharing), while using a NUMA node (with `-numa node,memdev=mem`) works just fine.
Steps to reproduce:
1. Install `virtiofsd`
2. Run `/usr/libexec/virtiofsd --socket-path=/tmp/vhostqemu --shared-dir=$HOME --cache always` as a regular user (or with another shared directory, it doesn't matter)
3. Run QEMU with the aforementioned command line as a regular user
4. Wait a bit for the OS to load and `virtiofsd` should error out
Additional information:
`virtiofsd` logs:
```
[2024-05-09T19:49:15Z WARN  virtiofsd::sandbox] Couldn't set the process uid as root: -1
[2024-05-09T19:49:15Z WARN  virtiofsd::sandbox] Couldn't set the process gid as root: -1
[2024-05-09T19:49:15Z INFO  virtiofsd] Waiting for vhost-user socket connection...
[2024-05-09T19:49:16Z INFO  virtiofsd] Client connected, servicing requests
[2024-05-09T19:49:22Z ERROR virtiofsd] Waiting for daemon failed: HandleRequest(ReqHandlerError(Custom { kind: Other, error: MissingMemoryMapping }))
```

QEMU logs (after virtiofsd errors out and exits):
```
qemu: Failed to read msg header. Read -1 instead of 12. Original request 0.
qemu: Failed to write msg. Wrote -1 instead of 20.
qemu: vhost VQ 1 ring restore failed: -22: Invalid argument (22)
qemu: Failed to set msg fds.
qemu: vhost VQ 0 ring restore failed: -22: Invalid argument (22)
qemu: Error starting vhost: 5
qemu: Failed to set msg fds.
qemu: vhost_set_vring_call failed 22
qemu: Failed to set msg fds.
qemu: vhost_set_vring_call failed 22
qemu: Unexpected end-of-file before all data were read
```