core: Fix regressions of generic root cell shrinking

This commit restores unmapping of a new cell's memory regions from the
root cell. Two small but fatal bugs broke it:
 - inverted error check in arch_unmap_memory_region (regression of
   738dffd234)
 - instead of setting mem.virt_start from phys_start, as stated in the
   comment, we did the opposite (regression of a033f90c8b)

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>