ch: virCHProcessSetupIOThreads: use correct type for return value

ch: virCHProcessSetupIOThreads: use correct type for return value

virCHMonitorGetIOThreads returns an int, not size_t.

Also return early if it's negative, because promoting it to
an unsigned type in the for loop condition could lead to
an infinte loop.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>