KVM: set_memory_region: Refactor commit_memory_region()

This patch makes the parameter old a const pointer to the old memory
slot and adds a new parameter named change to know the change being
requested: the former is for removing extra copying and the latter is
for cleaning up the code.

Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>