conf: Move iothread formatter into a separate function

conf: Move iothread formatter into a separate function

Formatting iothreads is currently open coded inside of
virDomainDefFormatInternalSetRootName(). While this works, it
makes the function needlessly long, especially if the formatting
code will expand in near future. Therefore, move it into a
separate function. At the same time, make
virDomainDefIothreadShouldFormat() accept const domain definition
so that the new function can also accept const domain definition.
Formatters shouldn't need to change definition.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>