fuzz: fuzz offsets within pio/mmio regions

The code did not add offsets to FlatRange bases, so we did not fuzz
offsets within device MemoryRegions.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20201029172901.534442-4-alxndr@bu.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>