hyperv/storvsc: Force SPC3 for CDROM attached.

This unbreaks the CDROM attaching on GEN2 VMs. On GEN1 VMs, CDROM is
attached to emulated ATA controller.

PR: 220790
Submitted by: Hongjiang Zhang <honzhan microsoft com>
MFC after: 3 days
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D11634