qemu: Always format CPU topology

qemu: Always format CPU topology

When libvirt cannot find a suitable CPU model for host CPU (easily
reproducible by running libvirt in a guest), it would not provide CPU
topology in capabilities XML either. Even though CPU topology is known
and can be queried by virNodeGetInfo. With this patch, CPU topology will
always be provided in capabilities XML regardless on the presence of CPU
model.
(cherry picked from commit f1c70100409562c3f402392aa667732e5f89a2c4)

Conflicts:

    src/qemu/qemu_capabilities.c
    src/qemu/qemu_command.c

    The new code uses capabilities caching.