vmx: Fix handling of FS/GS base MSRs.

vmx: Fix handling of FS/GS base MSRs.

Firstly, these MSRs are always accessible if the CPU supports them --
we should not check for EFER.LMA.

Secondly, we should not use teh cached value of shadow_gs while the
VCPU is running. It can be stale if the guest has executed SWAPGS
(which we cannot trap). Hence always access the underlying host MSR
when emulating guest accesses.

The latter bug was found and a patch proposed by <leonid@3tera.com>

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>