kvm: x86: Fix segment registers to make them VMX compatible

kvm: x86: Fix segment registers to make them VMX compatible

There are cases when the segment registers in gem5 are not compatible
with VMX. This changeset works around all known such issues. Specifically:

* The accessed bits in CS, SS, DD, ES, FS, GS are forced to 1.
* The busy bit in TR is forced to 1.
* The protection level of SS is forced to the same protection level as
  CS. The difference /seems/ to be caused by a bug in gem5's x86
  implementation.