Add Rom_session::update

The new Rom_session::update function can be used to request the update of
an existing ROM dataspace. If the new data fits into the existing
dataspace, a subsequent call of 'dataspace' can be omitted. This way,
ROM dataspace updates don't suffer from page-fault-handling costs that
would occur when replacing the dataspace with each update.
fixup desktop (light-weight focus model update)