qcow2: Support updating driver-specific options in reopen

qcow2: Support updating driver-specific options in reopen

For updating the cache sizes, disabling lazy refcounts and updating the
clean_cache_timer there is a bit more to do than just changing the
variables, but otherwise we're all set for changing options during
bdrv_reopen().

Just implement the missing pieces and hook the functions up in
bdrv_reopen().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>