virt-admin: Output srv-threadpool-info data as unsigned int rather than signed

virt-admin: Output srv-threadpool-info data as unsigned int rather than signed

Internally, all the data are represented as unsigned int, it is also documented
in the header file that users should use our exported constants that also
indicate that the data should be unsigned int. However, when polling for the
current server threadpool's configuration, virt-admin uses an incorrect
formatting parameter '%d' for printf. Instead, virt-admin should use formatting
parameter '%u'.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1356769

Signed-off-by: Erik Skultety <eskultet@redhat.com>