exec: Introduce AddressSpaceDispatch.mru_section

Under heavy workloads the lookup will likely end up with the same
MemoryRegionSection from last time. Using a pointer to cache the result,
like ram_list.mru_block, significantly reduces cost of
address_space_translate.

During address space topology update, as->dispatch will be reallocated
so the pointer is invalidated automatically.

Perf reports a visible drop on the cpu usage, because phys_page_find is
not called.  Before:

   2.35%  qemu-system-x86_64       [.] phys_page_find
   0.97%  qemu-system-x86_64       [.] address_space_translate_internal
   0.95%  qemu-system-x86_64       [.] address_space_translate
   0.55%  qemu-system-x86_64       [.] address_space_lookup_region

After:

   0.97%  qemu-system-x86_64       [.] address_space_translate_internal
   0.97%  qemu-system-x86_64       [.] address_space_lookup_region
   0.84%  qemu-system-x86_64       [.] address_space_translate

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1456813104-25902-8-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
