KVM: x86: hyperv: do rep check for each hypercall separately

Prepare to support TLB flush hypercalls, some of which are REP hypercalls.
Also, return HV_STATUS_INVALID_HYPERCALL_INPUT as it seems more
appropriate.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>