pc/vl: Add units-per-default-bus property

This patch adds the 'units_per_default_bus' property which
allows individual boards to declare their desired
index => (bus,unit) mapping for their default HBA, so that
boards such as Q35 can specify that its default if_ide HBA,
AHCI, only accepts one unit per bus.

This property only overrides the mapping for drives matching
the block_default_type interface.

This patch also adds this property to *all* past and present
Q35 machine types. This retroactive addition is justified
because the previous erroneous index=>(bus,unit) mappings
caused by lack of such a property were not utilized due to
lack of initialization code in the Q35 init routine.

Further, semantically, the Q35 board type has always had the
property that its default HBA, AHCI, only accepts one unit per
bus. The new code added to add devices to drives relies upon
the accuracy of this mapping. Thus, the property is applied
retroactively to reduce complexity of allowing IDE HBAs with
different units per bus.

Examples:

Prior to this patch, all IDE HBAs were assumed to use 2 units
per bus (Master, Slave). When using Q35 and AHCI, however, we
only allow one unit per bus.

-hdb foo.qcow2 would become index=1, or bus=0,unit=1.
-hdd foo.qcow2 would become index=3, or bus=1,unit=1.
-drive file=foo.qcow2,index=5 becomes bus=2,unit=1.

These are invalid for AHCI. They now become, under Q35 only:

-hdb foo.qcow2 --> index=1, bus=1, unit=0.
-hdd foo.qcow2 --> index=3, bus=3, unit=0.
-drive file=foo.qcow2,index=5 --> bus=5,unit=0.

The mapping is adjusted based on the fact that the default IF
for the Q35 machine type is IF_IDE, and units-per-default-bus
overrides the IDE mapping from its default of 2 units per bus
to just 1 unit per bus.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1412187569-23452-4-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
