XenAPI: Add versioning for plugins

Because the plugins live on a host seperate to Nova we need an interface
to test whether they are the expected version.  We can't use pip or other
requirement systems as they are cross-machine.

Closes bug 1226622

Change-Id: I58ab669061f51bd87071e2cf0d93d33021001309
(cherry picked from commit 24fd331b8cd5aca66362e6f76deec9c437875933)
Merge "XenAPI: Add versioning for plugins" into milestone-proposed