x86/hvm: Reposition the modification of raw segment data from the VMCB/VMCS

x86/hvm: Reposition the modification of raw segment data from the VMCB/VMCS

Intel VT-x and AMD SVM provide access to the full segment descriptor cache via
fields in the VMCB/VMCS.  However, the bits which are actually checked by
hardware and preserved across vmentry/exit are inconsistent, and the vendor
accessor functions perform inconsistent modification to the raw values.

Convert {svm,vmx}_{get,set}_segment_register() into raw accessors, and alter
hvm_{get,set}_segment_register() to cook the values consistently.  This allows
the common emulation code to better rely on finding architecturally-expected
values.

While moving the code performing the cooking, fix the %ss.db quirk.  A NULL
selector is indicated by .p being clear, not the value of the .type field.

This does cause some functional changes because of the modifications being
applied uniformly.  A side effect of this fixes latent bugs where
vmx_set_segment_register() didn't correctly fix up .G for segments, and
inconsistent fixing up of the GDTR/IDTR limits.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>