Allow more VMCB fields to be cached:

• CR2
• CR0, CR3, CR4 and EFER
• GDT/IDT base/limit fields
• CS/DS/ES/SS selector/base/limit/attrib fields

The caching can be further restricted via the tunable 'hw.vmm.svm.vmcb_clean'.

Restructure the code such that the fields above are only modified in a single
place. This makes it easy to invalidate the VMCB cache when any of these fields
is modified.