bhyve/virtio: use correct device id for virtio-scsi

Section 4.1.2.1 of the virtio spec states that the transitional PCI
device id for a scsi device is 0x1004.

Fix suggested by reporter.

PR: 259961
Reported by: me@nanaya.pro
Reviewed by: imp, jhb
Fixes: f9c005a17f4e ("Add bhyve virtio-scsi storage backend support.")
Differential Revision: https://reviews.freebsd.org/D34103