x86/emul: Fix emulation of RDSEED with older toolchains

This is reported as a MISRA R16.3 (missing break) violation, but turns out to
be substantially more complicated than expected.

In commit a8fe4ec532 ("x86emul: support RDRAND/RDSEED"), the switch()
statement had a default case going to cannot_emulate, with both the case 6 and
case 7 labels being fully within #ifdef HAVE_GAS_RD{RAND,SEED}.

Therefore, when the toolchain didn't understand the RDRAND/RDSEED
instructions, attempts to emulate them suffered #UD.  (In principle, this is a
problem as there's no interlock to prevent RDRAND/RDSEED being advertised to
the guest, but as instructions with only register encodings, they can only
legitimately be emulated when VM Introspection is in use.)

In commit 58f1bba440 ("x86emul: support RDPID"), case 7 was taken outside of
HAVE_GAS_RDSEED, meaning that emulating an RDSEED instruction no longer hit
the default case when the toolchain was too old.

Instead, it would fall out of the switch statement and be completed normally,
behaving as a NOP to the guest.

Retrofit a "return X86EMUL_UNIMPLEMENTED" in the case that the toolchain
doesn't know the RDRAND instruction, matching how RDRAND work.

Note that this has been fixed differently in Xen 4.21.  Commit
05bf9f1f0f ("x86/emulate: Remove HAVE_AS_RDRAND and HAVE_AS_RDSEED") has
removed the problematic condition due to the toolchain baseline upgrade.

Fixes: 58f1bba440 ("x86emul: support RDPID")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/x86_emulate/0fc7.c

@@ -102,6 +102,8 @@ int x86emul_0fc7(struct x86_emulate_state *s,
             if ( carry )
                 regs->eflags |= X86_EFLAGS_CF;
             break;
+#else
+            return X86EMUL_UNIMPLEMENTED;
 #endif
         }
     }