hw/pci: partially handle pci master abort

A MemoryRegion with negative priority was created and
it spans over all the pci address space.
It "intercepts" the accesses to unassigned pci
address space and will follow the pci spec:
 1. returns -1 on read
 2. does nothing on write

Note: setting the RECEIVED MASTER ABORT bit in the STATUS register
      of the device that initiated the transaction will be
      implemented in another series

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
