qcow2: Fix offset in qcow2_read_extensions

qcow2: Fix offset in qcow2_read_extensions

The spec says that the length of extensions is padded to 8 bytes, not
the offset. Currently this is the same because the header size is a
multiple of 8, so this is only about compatibility with future changes
to the header size.

While touching it, move the calculation to a common place instead of
duplicating it for each header extension type.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>