Allow xenguest to be disabled

• add a new --disable-xenguest argument

xenlight needs xen-4.4 and later

• only build xenlight on xen-4.4 and xen-4.5 (xen-4.2 is too old)

experimental xen-4.5 support

This resyncs the code with xen-unstable.

Bugfix release

• fix for HVM domain building on xen 4.2 and 4.3
• remove xen-{evtchn,gnt}: they now live in separate repos
• support for xen 4.4

Bugfix release

• Don't install the xenguest42 ocamlfind library -- it's only for internal use

More robust linking with xen-4.2

By moving the xenguest stubs into a shared library we fix the link order generated by OASIS.

Compiles on Debian/Ubuntu with xen-4.2

Merge pull request #35 from djs55/merge-euan

Merge Euan's fix for debian, regenerate OASIS