target/i386/kvm: Introduce i386_softmmu_kvm Meson source set

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>

target/i386/kvm/meson.build

 i386_ss.add(when: 'CONFIG_KVM', if_false: files('kvm-stub.c'))
 
-i386_softmmu_ss.add(when: 'CONFIG_KVM', if_true: files(
+i386_softmmu_kvm_ss = ss.source_set()
+
+i386_softmmu_kvm_ss.add(files(
   'kvm.c',
   'kvm-cpu.c',
 ))
 
 i386_softmmu_ss.add(when: 'CONFIG_HYPERV', if_true: files('hyperv.c'), if_false: files('hyperv-stub.c'))
+
+i386_softmmu_ss.add_all(when: 'CONFIG_KVM', if_true: i386_softmmu_kvm_ss)