qcow2: Rename l2_table in count_contiguous_clusters_unallocated()

qcow2: Rename l2_table in count_contiguous_clusters_unallocated()

This function doesn't need any changes to support L2 slices, but since
it's now dealing with slices instead of full tables, the l2_table
variable is renamed for clarity.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 78bcc54bc632574dd0b900a77a00a1b6ffc359e6.1517840877.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>