hw/i386: Deprecate -drive if=scsi with PC machine types

hw/i386: Deprecate -drive if=scsi with PC machine types

The PC machines (pc-q35-* pc-i440fx-* pc-* isapc xenfv) automatically
create lsi53c895a SCSI HBAs and SCSI devices to honor -drive if=scsi.
For giggles, try -drive if=scsi,bus=25,media=cdrom --- this makes QEMU
create 25 of them.

lsi53c895a is thoroughly obsolete (PCI Ultra2 SCSI, ca. 2000), and
currently has no maintainer in QEMU.  megasas is a better choice,
except with old OSes that lack drivers.  virtio-scsi is a much better
choice when you have a driver, but only (newish) Linux comes with one
in the box.  There is no good default that works for all guests.

Encourage users to pick a non-obsolete SCSI HBA that works for them by
deprecating -drive if=scsi.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487161136-9018-4-git-send-email-armbru@redhat.com>
Acked-By: Paolo Bonzini <pbonzini@redhat.com>