target/i386: Assert IOPL is 0 for user-only

target/i386: Assert IOPL is 0 for user-only

On real hardware, the linux kernel has the iopl(2) syscall which
can set IOPL to 3, to allow e.g. the xserver to briefly disable
interrupts while programming the graphics card.

However, QEMU cannot and does not implement this syscall, so the
IOPL is never changed from 0.  Which means that all of the checks
vs CPL <= IOPL are false for user-only.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210514151342.384376-9-richard.henderson@linaro.org>