x86: realmode: Test interrupt delivery after STI

If interrupts are disabled, STI is inhibiting interrupts for the
instruction following it. If STI is followed by HLT, the CPU is going to
handle all pending or new interrupts as soon as HLT is executed.

Test if emulator properly clears inhibition state and allows the
scenario outlined above.

Cc: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200329071125.79253-1-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>