CA-147820: don't run hotplug add if params is missing

This patch checks the return value of "xenstore-read" when it reads
"backend/vbd/domid/devid/params" while processing an "add" event. This
ensures that we won't write garbage to XenStore, which otherwise
confuses everyone.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Merge pull request #1969 from tmakatos/CA-147820

CA-147820: don't run hotplug add if params is missing
Whitespace: Tidy up update_table function

$ camlp4o -printer o -no_comments ocaml/xapi/export.ml | md5sum
fc1825315a9036f85b1741ffd30b64b0  -
$ git co HEAD^
$ camlp4o -printer o -no_comments ocaml/xapi/export.ml | md5sum
fc1825315a9036f85b1741ffd30b64b0  -

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Don't create refs for snapshots that don't exist on import

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Check that ref is valid, not just that it is not null

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #2040 from simonjbeaumont/sxm-failure-fix

Don't rewrite Refs for snapshots that don't exist
docs: fix typo

Signed-off-by: John Else <john.else@citrix.com>
docs: describe filtering out of dom0 display device

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