Relax xencenter_min_verstring

Blocking XenCenter 2.0 means that Clearwater XenCenter will fail to
reconnect to a server after installing the vGPU hotfix.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1564 from johnelse/relax-xencenter-min-version

Relax xencenter_min_verstring