Hello,

Attached for review is an implementation of migration to/from a file.

migrate-to-file: uses posix-aio-compat and FdMigration
migrate-from-file: uses qemu-fopen directly.

This patch is incomplete as currently it uses polling of aio-completion (we fake 
EAGAIN, and select finds the fd writeable, so file_write will be called again).

As Anthony suggested I plan to implement s->set_fd in FdMigration code, and use 
signal based aio-completion notifications (SIGUSR1).
Anthony mentioned we need only a single aio operation at a time. Although I'm 
not sure what the reasoning for that, I modified the code to support a single 
operation (my previous implementation used multiple aio operations).

Differences from v2:
    use aio writes (posix-aio-compat) instead of select and/or write.
    compile migration-to-file only if AIO is configured.

An alternative (v4) would be to use a single thread and a pipe which is also 
asynchronous, much easier to implement and possibly faster (need to be measured).

Regards,
     Uri.