hw: Default -drive to if=ide explicitly where it works

Block backends defined with -drive if=ide are meant to be picked up by
machine initialization code: a suitable frontend gets created and
wired up automatically.

if=ide drives not picked up that way can still be used with -device as
if they had if=none, but that's unclean and best avoided.  Unused ones
produce an "Orphaned drive without device" warning.

-drive parameter "if" is optional, and the default depends on the
machine type.  If a machine type doesn't specify a default, the
default is "ide".

Many machine types default to if=ide, even though they don't actually
have an IDE controller.  A future patch will change these defaults to
something more sensible.  To prepare for it, this patch makes default
"ide" explicit for the machines that actually pick up if=ide drives:

* alpha: clipper
* arm/aarch64: spitz borzoi terrier tosa
* i386/x86_64: generic-pc-machine (with concrete subtypes pc-q35-*
  pc-i440fx-* pc-* isapc xenfv)
* mips64el: fulong2e
* mips/mipsel/mips64el: malta mips
* ppc/ppc64: mac99 g3beige prep
* sh4/sh4eb: r2d
* sparc64: sun4u sun4v

Note that ppc64 machine powernv already sets an "ide" default
explicitly.  Its IDE controller isn't implemented, yet.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1487153147-11530-2-git-send-email-armbru@redhat.com>

hw/mips/mips_malta.c

@@ -1264,6 +1264,7 @@ static void mips_malta_machine_init(MachineClass *mc)
 {
     mc->desc = "MIPS Malta Core LV";
     mc->init = mips_malta_init;
+    mc->block_default_type = IF_IDE;
     mc->max_cpus = 16;
     mc->is_default = 1;
 }