commit 1a18a69b upstream.

If the guest thinks it's an AMD, it will not have prepared the SYSENTER MSRs,
and if the guest executes SYSENTER in compatibility mode, it will fails.

Detect this condition and #UD instead, like the spec says.
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>