selftests/x86/entry_from_vm86: Add test cases for POPF

POPF is currently broken -- add tests to catch the error.  This
results in:

   [RUN]	POPF with VIP set and IF clear from vm86 mode
   [INFO]	Exited vm86 mode due to STI
   [FAIL]	Incorrect return reason (started at eip = 0xd, ended at eip = 0xf)

because POPF currently fails to check IF before reporting a pending
interrupt.

This patch also makes the FAIL message a bit more informative.

Reported-by: Bart Oldeman <bartoldeman@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stas Sergeev <stsp@list.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/a16270b5cfe7832d6d00c479d0f871066cbdb52b.1521003603.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>