cpu-plug-test: Don't pass integers as strings to device_add

cpu-plug-test: Don't pass integers as strings to device_add

test_plug_with_device_add_x86() plugs Haswell-i386-cpu and
Haswell-x86_64-cpu with device_add.  It passes socket-id, core-id,
thread-id as JSON strings.  The properties are actually integers.

test_plug_with_device_add_coreid() plugs power8_v2.0-spapr-cpu-core
and qemu-s390x-cpu with device_add.  It passes core-id as JSON string.
The properties are actually integers.

Passing JSON string values to integer properties works only due to
device_add implementation accidents.  Fix the test to pass JSON
numbers.  While there, use %u rather than %i with unsigned int.

Cc: Thomas Huth <thuth@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-14-armbru@redhat.com>