Whitespace: Xapi_pv_drivers_version.of_drivers_version.

$ camlp4o -printer o -no_comments ocaml/xapi/xapi_pv_driver_version.ml | md5sum
5605b59b4dae1e43796c0b158ae99dcd  -
$ git checkout HEAD^
$ camlp4o -printer o -no_comments ocaml/xapi/xapi_pv_driver_version.ml | md5sum
5605b59b4dae1e43796c0b158ae99dcd  -

Signed-off-by: John Else <john.else@citrix.com>
CA-89023: Fix PV tools OS type detection.

Using the /drivers/xenvbd xenstore key to flag that a guest is Windows
is problematic, since:

1. This key goes away after suspend/resume or migrate, so cannot be
   relied upon to mark PV tools as out of date after a rolling pool upgrade.
2. This key isn't written at all by the Tampa PV drivers.

Instead, we now check that /data/os_distro equals "windows" - this value is
hardcoded in the windows guest agent.

Signed-off-by: John Else <john.else@citrix.com>
Fix typo.

Signed-off-by: John Else <john.else@citrix.com>