qcow2: add option to clean unused cache entries after some time

qcow2: add option to clean unused cache entries after some time

This adds a new 'cache-clean-interval' option that cleans all qcow2
cache entries that haven't been used in a certain interval, given in
seconds.

This allows setting a large L2 cache size so it can handle scenarios
with lots of I/O and at the same time use little memory during periods
of inactivity.

This feature currently relies on MADV_DONTNEED to free that memory, so
it is not useful in systems that don't follow that behavior.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: a70d12da60433df9360ada648b3f34b8f6f354ce.1438690126.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>