x86/sev: Disable MMIO emulation from user mode

A virt scenario can be constructed where MMIO memory can be user memory.
When that happens, a race condition opens between when the hardware
raises the #VC and when the #VC handler gets to emulate the instruction.

If the MOVS is replaced with a MOVS accessing kernel memory in that
small race window, then write to kernel memory happens as the access
checks are not done at emulation time.

Disable MMIO emulation in user mode temporarily until a sensible use
case appears and justifies properly handling the race window.

Fixes: 0118b604 ("x86/sev-es: Handle MMIO String Instructions")
Reported-by: Tom Dohrmann <erbse.13@gmx.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Tom Dohrmann <erbse.13@gmx.de>
Cc: <stable@kernel.org>