powerpc/configs: Incorporate generic kvm_guest.config into guest configs

Incorporate the generic kvm_guest.config into the powerpc guest configs,
ppc64[le]_guest_defconfig.

This brings in some useful options, in particular 9P support, and also
means future additions to the generic file will be automatically picked
up by the powerpc configs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230414132415.821564-30-mpe@ellerman.id.au