Implement support for a "file:" uri so that a migration can be initiated
directly to a file from QEMU.
Unlike other migration protocol backends, the 'file' protocol cannot
honour non-blocking mode. POSIX file/block storage will always report
ready to read/write, regardless of how slow the underlying storage
will be at servicing the request.
For outgoing migration this limitation is not a serious problem as
the migration data transfer always happens in a dedicated thread.
It may, however, result in delays in honouring a request to cancel
the migration operation.
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
4ed18df9df