powerpc/configs: Add IBMVETH=y and IBMVNIC=y to guest configs

These drivers are sometimes required to have functional networking in a
guest, so build them in when building ppc64[le]_guest_defconfig.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230414132415.821564-29-mpe@ellerman.id.au