qdev: Move global validation to a single function

Currently GlobalProperty.not_used=false has multiple meanings:

* It may be a property for a hotpluggable device, which may or may not
  have been used by a device;
* It may be a machine-type-provided property, which may or may not have
  been used by a device.
* It may be a user-provided property that was actually not used by
  any device.

Simplify the logic by having two separate fields: 'user_provided' and
'used'. This allows the entire global property validation logic to be
contained in a single function, and allows more specific error messages.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
