ppc: Fix build with --without-default-devices

ppc: Fix build with --without-default-devices

Linking of the qemu-system-ppc64 fails on a POWER9 host when
--without-default-devices is passed to configure:

$ ./configure --without-default-devices \
              --target-list=ppc64-softmmu && make

...

libqemu-ppc64-softmmu.fa.p/hw_ppc_e500.c.o: In function `ppce500_init_mpic_kvm':
/home/greg/Work/qemu/qemu-ppc/build/../hw/ppc/e500.c:777: undefined reference to `kvm_openpic_connect_vcpu'
libqemu-ppc64-softmmu.fa.p/hw_ppc_spapr_irq.c.o: In function `spapr_irq_check':
/home/greg/Work/qemu/qemu-ppc/build/../hw/ppc/spapr_irq.c:189: undefined reference to `xics_kvm_has_broken_disconnect'
libqemu-ppc64-softmmu.fa.p/hw_intc_spapr_xive.c.o: In function `spapr_xive_post_load':
/home/greg/Work/qemu/qemu-ppc/build/../hw/intc/spapr_xive.c:530: undefined reference to `kvmppc_xive_post_load'

... and tons of other symbols belonging to the KVM backend of the
openpic, XICS and XIVE interrupt controllers.

It turns out that OPENPIC_KVM, XICS_KVM and XIVE_KVM are marked
to depend on KVM but this has no effect when minikconf runs in
allnoconfig mode. Such reverse dependencies should rather be
handled with a 'select' statement, eg.

config OPENPIC
    select OPENPIC_KVM if KVM

or even better by getting rid of the intermediate _KVM config
and directly checking CONFIG_KVM in the meson.build file:

specific_ss.add(when: ['CONFIG_KVM', 'CONFIG_OPENPIC'],
		if_true: files('openpic_kvm.c'))

Go for the latter with OPENPIC, XICS and XIVE.

This went unnoticed so far because CI doesn't test the build with
--without-default-devices and KVM enabled on a POWER host.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <160883056791.253005.14924294027763955653.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>