PIIX created a made-up value for the UP register since it was read by
guest 32 times for each interrupt.
There's no reason to do this for the new PCIHP: register is only read
once for each interrupt, so clean up code by making read act as an
interrupt acknowledgement: the new UP register clear on read.

In this way we cut down the number of bus rescans
by a factor of 32, and drop a bunch of code that's
now unused.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>