memory: separate building the final memory map into two steps

memory: separate building the final memory map into two steps

Instead of adding and deleting regions in one pass, do a delete
pass followed by an add pass.  This fixes the following case:

from:
  0x0000-0x0fff ram  (a1)
  0x1000-0x1fff mmio (a2)
  0x2000-0x2fff ram  (a3)

to:
  0x0000-0x2fff ram  (b1)

The single pass algorithm removed a1, added b2, then removed a2 and a3,
which caused the wrong memory map to be built.  The two pass algorithm
removes a1, a2, and a3, then adds b1.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>