We don't want to assume a contiguous address space, so migrate based
on RAM blocks instead of a fixed linear address map.  This will allow
us to have holes in the ram_addr_t namespace, so we can implement
qemu_ram_free().

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>