Change the prototypes of vdsk_readv() and vdsk_writev() so that they
don't return the amount read or written with -1 indicating an error,
but rather just return the error. We don't support partial reads or
partial writes, so trying to be like the quirky POSIX interface is
just quirky.

Rename the functions by dropping the v suffix. We're less like preadv
and pwritev now than before and we don't have the non-iovec variants
to distinguish from.