pseries: Add a routine to find a stable "default" vty and use it

In vty_lookup() we have a special case for supporting early debug in
the kernel. This accepts reg == 0 as a special case to mean "any vty".

We implement this by searching the vtys on the bus and returning the
first we find. This means that the vty we chose depends on the order
the vtys are specified on the QEMU command line - because that determines
the order of the vtys on the bus.

We'd rather the command line order was irrelevant, so instead return
the vty with the lowest reg value. This is still a guess as to what the
user really means, but it is at least stable WRT command line ordering.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>

[agraf] fix braces

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