capabilities: Separate <cpu/> formatting into a function

capabilities: Separate <cpu/> formatting into a function

The way we format <cpu/> element for capabilities is not ideal,
because if there are no CPUs, i.e. no child elements, we still
output opening and closing element. To solve this,
virXMLFormatElement() could be used but that would introduce more
variables into the loop. Therefore, move the formatter into a
separate function and use virXMLFormatElement().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>