hw/vfio: fix region fd initialization

hw/vfio: fix region fd initialization

We were not initializing the region fd array to -1, so we would
accidentally try to close(0) on cleanup for any region that is not
referenced.

Fixes: 95cdb024 ("vfio: add region info cache")
Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250715115954.515819-3-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>