xenapi:populating hypervisor version in host state

This will be used in the scheduler to compare it to the image
properties.
Pulled out conversion of version string into integer to utils.
Partially implements blueprint xen-support-for-hypervisor-versions

Change-Id: Ib2eb91f6f18af6d39f9381daeff7ba9a24b4785a
Merge "xenapi:populating hypervisor version in host state"