smbios: Set system manufacturer, product & version by default

smbios: Set system manufacturer, product & version by default

Currently, we get SeaBIOS defaults: manufacturer Bochs, product Bochs,
no version.  Best SeaBIOS can do, but we can provide better defaults:
manufacturer QEMU, product & version taken from QEMUMachine desc and
name.

Take care to do this only for new machine types, of course.

Note: Michael Tsirkin doesn't trust us to keep values of QEMUMachine member
product stable in the future.  Use copies instead, and in a way that
makes it obvious that they're guest ABI.

Note that we can be trusted to keep values of member name, because
that has always been ABI.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>