qdev: support properties which don't set a default value

qdev: support properties which don't set a default value

In some situations it's useful to have a qdev property which doesn't
automatically set its default value when qdev_property_add_static is
called (for instance when the default value is not constant).

Support this by adding a flag to the Property struct indicating
whether to set the default value.  This replaces the existing test
for whether the PropertyInfo set_default_value function pointer is
NULL, and we set the .set_default field to true for all those cases
of struct Property which use a PropertyInfo with a non-NULL
set_default_value, so behaviour remains the same as before.

This gives us the semantics of:
 * if .set_default is true, then .info->set_default_value must
   be not NULL, and .defval is used as the the default value of
   the property
 * otherwise, the property system does not set any default, and
   the field will retain whatever initial value it was given by
   the device's .instance_init method

We define two new macros DEFINE_PROP_SIGNED_NODEFAULT and
DEFINE_PROP_UNSIGNED_NODEFAULT, to cover the most plausible use cases
of wanting to set an integer property with no default value.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1499788408-10096-3-git-send-email-peter.maydell@linaro.org