KVM: VMX: Cache vmcs segment fields

Since the emulator now checks segment limits and access rights, it
generates a lot more accesses to the vmcs segment fields.  Undo some
of the performance hit by cacheing those fields in a read-only cache
(the entire cache is invalidated on any write, or on guest exit).

Signed-off-by: Avi Kivity <avi@redhat.com>