Bug fix for DR_task.create.
Bug fix for DR_task.create.

When creating a DR_task object, if the task finds an SR whose UUID is on
the whitelist but also already present in the database, instead of a
Uniqueness_constraint_violation being thrown the DR_task now logs a
message and continues without doing anything to the already-introduced SR.

Signed-off-by: John Else <37b74501daa612e48fea94f9846ed4ddeb6a8f07@citrix.com>

djs55/xcp-networkd,koushikcgit/xcp-rrdd,koushikcgit/xcp-rrdd,djs55/xcp-networkd,johnelse/xcp-rrdd,koushikcgit/xcp-rrdd,simonjbeaumont/xcp-rrdd,johnelse/xcp-rrdd,djs55/xcp-rrdd,koushikcgit/xcp-networkd,sharady/xcp-networkd,djs55/xcp-rrdd,djs55/squeezed,koushikcgit/xcp-networkd,robhoes/squeezed,simonjbeaumont/xcp-rrdd,sharady/xcp-networkd