qcow2: Let inc_refcounts() resize the reftable

Now that the refcount table can be passed around by reference, do that
for inc_refcounts() (and subsequently check_refcounts_l1() and
check_refcounts_l2()) and use it for resizing it when a cluster after
the image end is encountered.

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