powerpc/kvm: update KVM config options in ppc44x_defconfig

KVM host support was recently enabled in ppc44x_defconfig, but since then the
config option was renamed. Update ppc44x_defconfig to match.

Also, KVM guests aren't very interesting without networking, so enable
CONFIG_TUN and CONFIG_BRIDGE.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>