crosvm: guest_memory: add memory_size method
crosvm: guest_memory: add memory_size method

This is useful to get the total size of memory without having to write
something that iterates over the regions explicitly.

BUG=chromium:797868
TEST=./build_test passes on all architectures
TEST=crosvm runs on caroline

Change-Id: Iac9a341b4c41d6462cf731f6267b92a0169578e4
Reviewed-on: https://chromium-review.googlesource.com/977565
Commit-Ready: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Tested-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm