CP-8744:  SR trim limited to compatible SRs

Limited SR trim capability to LVHD based SRs. Also error handling
was modified to return a XML string response on failure containing
errcode and errmsg as a key value pair. Trim functionality was
separated from the trim plugin entry point.

Signed-off-by: Vineeth Raveendran <vineeth.thampi@citrix.com>
Reviewed-by: Mate Lakat <mate.lakat@citrix.com>

GitHub: closes #182 xapi-project/sm
CP-8744: [UnitTest] Add unittest for trim plugin

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
Reviewed-by: Vineeth Raveendran <vineeth.thampi@citrix.com>