x86/sev-es: Handle MMIO String Instructions

Add handling for emulation of the MOVS instruction on MMIO regions, as
done by the memcpy_toio() and memcpy_fromio() functions.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200907131613.12703-51-joro@8bytes.org