commit 378a8b09 upstream.

If VMX reports segment as unusable, zero descriptor passed by the emulator
before returning. Such descriptor will be considered not present by the
emulator.
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>