KVM: cpuid: set CPUID(EAX=0xd,ECX=1).EBX correctly

This is the size of the XSAVES area.  This starts providing guest support
for XSAVES (with no support yet for supervisor states, i.e. XSS == 0
always in guests for now).

Wanpeng Li suggested testing XSAVEC as well as XSAVES, since in practice
no real processor exists that only has one of them, and there is no
other way for userspace programs to compute the area of the XSAVEC
save area.  CPUID(EAX=0xd,ECX=1).EBX provides an upper bound.

Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Tested-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>