from DIO_REQUEST. Also do_vdevio. Also do_sdevio, but this
function also supports grant id's and offsets.
do_segctl: rename protected to prot.
do_umap: support for GRANT_SEG umap.
do_privctl: support SYS_PRIV_SET_GRANTS, which sets location and size
of in-own-address-space grant table.
do_safecopy: functions to verify and perform 'safe' (grant-based) copies.
