Correct API versioning for the vdi.copy extension

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Bump API version to 2.2

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
datamodel: VDI.copy was extended in clearwater-felton, not clearwater

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
README: correct links (should be xenproject.org, not xen.org)

Signed-off-by: David Scott <dave.scott@citrix.com>
README: don't use "XCP" any more; don't use opam; point people at xenserver-core

The XCP story is mostly historical and quite confusing.
The opam build instructions don't work but the xenserver-core ones
are quite good.

Signed-off-by: David Scott <dave.scott@citrix.com>
Add comments explaining xencenter_min_verstring and xencenter_max_verstring
Merge pull request #1680 from djs55/correct-list-address

Update the very old README
Merge pull request #1681 from stephen-turner/patch-1

Add comments explaining xencenter_min_verstring and xencenter_max_verstring
Merge pull request #1673 from robhoes/master

Update API versioning in datamodel to be consistent with CW