block/blklogwrites: Add an option for the update interval of the log superblock

This is a way to ensure that the log superblock is periodically
updated. Before, this was only done on flush requests, which may
not be enough if the VM exits abnormally, omitting the final flush.

The default interval is 4096 write requests.

Signed-off-by: Ari Sundholm <ari@tuxera.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>