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:             [[ https://bugs.freebsd.org/220790 | 220790 ]]
Submitted by:   Hongjiang Zhang <honzhan microsoft com>
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D11634