qom: Document qom/device-list-properties implementation specific

The recently introduced qom-list-properties QMP command raised
a question what properties it (and its cousin - device-list-properties)
can possibly print - only those defined by DeviceClass::props
or dynamically created in TypeInfo::instance_init() so properties created
elsewhere won't show up and this behaviour might confuse the user.

For example, PIIX4 does that from piix4_pm_realize() via
  piix4_pm_add_propeties():

object_property_add_uint8_ptr(OBJECT(s), ACPI_PM_PROP_ACPI_ENABLE_CMD,
                              &acpi_enable_cmd, NULL);

This adds a note to the command descriptions about the limitation.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20180530071129.9013-1-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>