KVM/x86: Move X86_CR4_OSXSAVE check into kvm_valid_sregs()

X86_CR4_OSXSAVE check belongs to sregs check and so move into
kvm_valid_sregs().

Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>