dump: populate guest_phys_blocks

While the machine is paused, in guest_phys_blocks_append() we register a
one-shot MemoryListener, solely for the initial collection of the valid
guest-physical memory ranges that happens at listener registration time.

For each range that is reported to guest_phys_blocks_region_add(), we
attempt to merge the range with the preceding one.

Ranges can only be joined if they are contiguous in both guest-physical
address space, and contiguous in host virtual address space.

The "maximal" ranges that remain in the end constitute the guest-physical
memory map that the dump will be based on.

Related RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=981582

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
