block: complete all IOs before .bdrv_truncate

bdrv_truncate() invalidates the bdrv_check_request() result for
in-flight requests, so there should better be none.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Reported-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
