i386: Enable TOPOEXT feature on AMD EPYC CPU

Enable TOPOEXT feature on EPYC CPU. This is required to support
hyperthreading on VM guests. Also extend xlevel to 0x8000001E.

Disable topoext on PC_COMPAT_2_12 and keep xlevel 0x8000000a.

Signed-off-by: Babu Moger <babu.moger@amd.com>
Message-Id: <1529443919-67509-3-git-send-email-babu.moger@amd.com>
[ehabkost: Added EPYC-IBPB.xlevel to PC_COMPAT_2_12]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>