While not mandated by the spec, Linux relies on NMI being blocked by an
IF-enabling STI.  VMX also refuses to enter a guest in this state, at
least on some implementations.

Disallow NMI while blocked by STI by checking for the condition, and
requesting an interrupt window exit if it occurs.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>