ch: Use int for @niothreads in virCHMonitorGetIOThreads()

ch: Use int for @niothreads in virCHMonitorGetIOThreads()

The @niothreads inside of virCHMonitorGetIOThreads() is declared
as of size_t type. This would work, except the variable is then
passed to VIR_DEBUG with incorrect format directive (%ld) and
returned.  But the function returns an int not size_t. Fix the
variable declaration and format directive.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>