KVM: x86: mask unsupported XSAVE entries from leaf 0Dh index 0

XSAVE entries that KVM does not support are reported by
KVM_GET_SUPPORTED_CPUID for leaf 0Dh index 0 if the host supports them;
they should be left out unless there is also hypervisor support for them.

Sub-leafs are correctly handled in supported_xcr0_bit, fix index 0
to match.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>