x86: Add EFER descriptions for FFXSR

AMD k10 includes support for the FFXSR feature, which leaves out
XMM registers on FXSAVE/FXSAVE when the EFER_FFXSR bit is set in
EFER.

The CPUID feature bit exists already, but the EFER bit is missing
currently, so this patch adds it to the list of known EFER bits.

Signed-off-by: Alexander Graf <agraf@suse.de>
CC: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>