tests: cpu-plug-test: fix device_add for pc/q35 machines

tests: cpu-plug-test: fix device_add for pc/q35 machines

Commit bc1fb850a3 silently broke device_add test for CPU hotplug which
resulted in test successfully passing though it wasn't actually run.
Fix it by making sure that all non present CPUs reported
by "query-hotpluggable-cpus" are hotplugged instead of making up
and hardcoding values.

Use of query-hotpluggable-cpus also allows consolidatiate device_add
cpu testcases and reuse the same test function for all targets.

While at it also add a check that at least one CPU was hotplugged,
to avoid silent breakage in the future.

Fixes: bc1fb850a3 (vl.c deprecate incorrect CPUs topology)
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20190830110723.15096-3-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>