Support default block interfaces per QEMUMachine

Support default block interfaces per QEMUMachine

There are QEMUMachines that have neither IF_IDE nor IF_SCSI as a
default/standard interface to their block devices / drives. Therefore,
this patch introduces a new field default_block_type per QEMUMachine
struct. The prior use_scsi field becomes thereby obsolete and is
replaced through .default_block_type = IF_SCSI.

This patch also changes the default for s390x to IF_VIRTIO and
removes an early hack that converts IF_IDE drives.
Other parties have already claimed interest (e.g. IF_SD for exynos)

To create a sane default, for machines that dont specify a
default_block_type, this patch makes IF_IDE = 0 and IF_NONE = 1.
I checked all users of IF_NONE (blockdev.c and ww/device-hotplug.c)
as well as IF_IDE and it seems that it is ok to change the defines -
in other words, I found no obvious (to me) assumption in the code
regarding IF_NONE==0. IF_NONE is only set if there is an
explicit if=none. Without if=* the interface becomes IF_DEFAULT.

I would suggest to have some additional care, e.g. by letting
this patch sit some days in the block tree.

Based on an initial patch from Einar Lueck <[email protected]>

Signed-off-by: Christian Borntraeger <[email protected]>
CC: Igor Mitsyanko <[email protected]>
CC: Markus Armbruster <[email protected]>
CC: Kevin Wolf <[email protected]>
Reviewed-by: Alexander Graf <[email protected]>
Acked-by: Igor Mitsyanko <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>