Introduce the i386_softmmu_kvm Meson source set to be able to
add features dependent on CONFIG_KVM.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211007161716.453984-4-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>