memory: add a sparse memory device for fuzzing

memory: add a sparse memory device for fuzzing

For testing, it can be useful to simulate an enormous amount of memory
(e.g. 2^64 RAM). This adds an MMIO device that acts as sparse memory.
When something writes a nonzero value to a sparse-mem address, we
allocate a block of memory. For now, since the only user of this device
is the fuzzer, we do not track and free zeroed blocks. The device has a
very low priority (so it can be mapped beneath actual RAM, and virtual
device MMIO regions).

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