kvm: Enable -cpu option to hide KVM

The latest Nvidia driver (337.88) specifically checks for KVM as the
hypervisor and reports Code 43 for the driver in a Windows guest when
found.  Removing or changing the KVM signature is sufficient for the
driver to load and work.  This patch adds an option to easily allow
the KVM hypervisor signature to be hidden using '-cpu kvm=off'.  We
continue to expose KVM via the cpuid value by default.  The state of
this option does not supercede or replace -enable-kvm or the accel=kvm
machine option.  This only changes the visibility of KVM to the guest
and paravirtual features specifically tied to the KVM cpuid.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
