[Feature Request] Support non-seekable file descriptors (e.g. FIFOs) for snapshot memory output

## Motivation

It would be useful to point `mem_file_path` at a FIFO (named pipe) so snapshot memory can be streamed to another process (e.g. compression, encryption, or upload) without first landing on local disk.

Currently this doesn't work because `dump()`/`dump_dirty()` in `src/vmm/src/vstate/memory.rs` require the writer to implement `Seek`, and `snapshot_memory_to_file` in `src/vmm/src/vstate/vm.rs` calls `file.set_len()` before writing. Both operations fail on non-seekable file descriptors (`ESPIPE`).

Worse, the failure is silent — when `mem_file_path` is a FIFO, the `CreateSnapshot` API hangs indefinitely instead of returning an error. The `.unwrap()` on the seek call causes a panic or unrecoverable internal state:

```rust
fn dump(&self, writer: &mut T) -> Result<(), MemoryError> {
self.iter()
.flat_map(|region| region.slots())
.try_for_each(|(mem_slot, plugged)| {
if !plugged {
let ilen = i64::try_from(mem_slot.slice.len()).unwrap();
writer.seek(SeekFrom::Current(ilen)).unwrap(); // panics on FIFO
} else {
writer.write_all_volatile(&mem_slot.slice)?;
}
Ok(())
})
.map_err(MemoryError::WriteMemory)
}
```

## Describe the desired solution

Two separable improvements:

1. Fail fast: If mem_file_path is not a regular file (or if set_len() / seek() fails), return a clear API error instead of hanging.
2. Support non-seekable fds: For unplugged memory slots, dump() uses seek(SeekFrom::Current(n)) to skip over them. On a non-seekable fd, this could be replaced with writing n zero bytes. The set_len() pre-allocation could be skipped when the target is not a regular file.

2. The diff-snapshot path (dump_dirty) has the same pattern — skip clean pages via seek, write dirty pages — and could be adapted similarly.

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