hw/pci: Use -1 as the default value for rombar

hw/pci: Use -1 as the default value for rombar

vfio_pci_size_rom() distinguishes whether rombar is explicitly set to 1
by checking dev->opts, bypassing the QOM property infrastructure.

Use -1 as the default value for rombar to tell if the user explicitly
set it to 1. The property is also converted from unsigned to signed.
-1 is signed so it is safe to give it a new meaning. The values in
[2 ^ 31, 2 ^ 32) become invalid, but nobody should have typed these
values by chance.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250104-reuse-v18-13-c349eafd8673@daynix.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>