Add FIST capability for pools

By setting xenstore parameters 'fist/pool/<name>/defer' to X and
'fist/pool/<name>/probability' to Y you can now cause failures of PoolGet()
such that it will fail with probability Y% after X initial invocations.
The name of the grant table pool has been set to 'gnttab' to make typing it
in more convenients, and some debug-keys output has been added to give
the PoolGet() failure count.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Merge branch 'master' of github.com:xenserver/win-xenbus
Merge pull request #22 from pauldu/fist

FIST

Approved-By: Ben Chalmers <Ben.Chalmers@citrix.com>