arch-x86: Detect when entering virtual 8086 mode.

If we're entering virtual 8086 mode, panic. Some aspects of that mode
may actually work, but since it's essentially untested, lets be extra
cautious.

Change-Id: I78bbfcb75db8370f4271c75caabc0ec53f75a884
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55810
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/x86/isa.cc

@@ -318,6 +318,12 @@ ISA::setMiscReg(int miscReg, RegVal val)
         break;
       case MISCREG_CR8:
         break;
+      case MISCREG_RFLAGS:
+        {
+            RFLAGS rflags = val;
+            panic_if(rflags.vm, "Virtual 8086 mode is not supported.");
+            break;
+        }
       case MISCREG_CS_ATTR:
         {
             SegAttr toggled = regVal[miscReg] ^ val;