RFC: xen: read entropy from the Xen entropy device

We connect to an entropy server over the well-known console channel

  org.openmirage.entropy.1

Limitations:

- we read exactly one block of entropy data. Should we read as much
  as the backend will offer? Note the backend implementation has a
  rate limiter to avoid starving dom0.
- there is no fall-back for cases where there is no Xen entropy source
  i.e. on the vast majority of Xen hosts including AWS.

Signed-off-by: David Scott <dave.scott@citrix.com>
I think the default 'weak' isn't that weak though -- we can implement a decent subset of what Amazon VMs already do to mix in entropy -- inter-ring packet times, etc.