file-posix: Make .bdrv_co_truncate asynchronous

This moves the code to resize an image file to the thread pool to avoid
blocking.

Creating large images with preallocation with blockdev-create is now
actually a background job instead of blocking the monitor (and most
other things) until the preallocation has completed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>