Kconfig: kvm: allow building without any board

Kconfig: kvm: allow building without any board

KVM code might have to call functions on the PCIDevice that is
passed to kvm_arch_fixup_msi_route().  This fails in the case
where --without-default-devices is used and no board is
configured.  While this is not really a useful configuration,
and therefore setting up stubs for CONFIG_PCI is overkill,
failing the build is impolite.  Just include the PCI
subsystem if kvm_arch_fixup_msi_route() requires it, as
is the case for ARM and x86.

Reported-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>