vl: disable default cdrom when using explicitely scsi-hd

In commit af6bf132 (May 2011),
ide-hd, ide-cd and scsi-cd have been added to disable default cdrom,
"or else you can't put one on secondary master without -nodefaults".

Make it the same for scsi-hd, so you can put one on scsi-id 2 without
using -nodefaults.
scsi-hd has probably been forgotten, as it has been added in the
preceding commit (b443ae67).

Affected users are the ones using a machine with SCSI devices and start QEMU
with -device scsi-hd but without -device scsi-cd or -cdrom
In that case, the default cdrom device will disappear instead of being empty.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Message-Id: <1487623279-29930-1-git-send-email-hpoussin@reactos.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>