kvm: svm: Update svm_xsaves_supported

AMD CPUs now support XSAVES in a limited fashion (they require IA32_XSS
to be zero).

AMD has no equivalent of Intel's "Enable XSAVES/XRSTORS" VM-execution
control. Instead, XSAVES is always available to the guest when supported
on the host.

Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Change-Id: I40dc2c682eb0d38c2208d95d5eb7bbb6c47f6317
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>