conf, schema: Switch iothread/poll values to unsignedLong

conf, schema: Switch iothread/poll values to unsignedLong

They represent nanoseconds, and we accept such values already.  Not that
anyone would use such values in the wild, but even one person testing
QEMU could put in a bigger value and will be bothered with validation
errors after every `virsh edit`.  Also add a test for it.

Resolves: https://issues.redhat.com/browse/RHEL-1717

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>