CA-135189: Remove restricted device_config on upgrade

The SRmaster key is restricted for internal use only. There are precautions
taken against creating PBDs with such a key in the device config now but we
should purge the database of any such keys created before these measures were
in place.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>

Conflicts:
	ocaml/xapi/xapi_db_upgrade.ml
Merge pull request #1738 from simonjbeaumont/ca-135189-creedence

[xs64bit] CA-135189: Remove restricted device_config on upgrade