As Avi correctly noted, last_ram_offset does not mark the last physical
RAM address the guest may see (due to non-continuous memory regions).
Ensure that we catch them all by marking the full possible address range
dirty.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>