Restructure the MSR handling so it is entirely handled by processor-specific
code. There are only a handful of MSRs common between the two so there isn't
too much duplicate functionality.

The VT-x code has the following types of MSRs:

- MSRs that are unconditionally saved/restored on every guest/host context
  switch (e.g., MSR_GSBASE).

- MSRs that are restored to guest values on entry to vmx_run() and saved
  before returning. This is an optimization for MSRs that are not used in
  host kernel context (e.g., MSR_KGSBASE).

- MSRs that are emulated and every access by the guest causes a trap into
  the hypervisor (e.g., MSR_IA32_MISC_ENABLE).

Reviewed by:	grehan