add an entropy device for mirage
fix typo here
type t = unit instead of string
rename error constructor
commit to a cstruct.t as buffer
use different module names for unix and xen
Sync changes
Allow the nature of the entropy device to be specified in the device

We classify this into `Strongest` for the best one, and `Strong`
and `Weak` if one is to be specifically selected.  The default is
`Strongest`, which maps to `/dev/random` on Unix and currently a
weak (but functioning) source on Xen.
Merge pull request #259 from avsm/hannesm-add-entropy-device

Updates to entropy device support