vm_memory: add GuestAddress::align
vm_memory: add GuestAddress::align

Originally added on Windows. Allows us to get an aligned address from a
GuestAddress.

TEST=builds
BUG=b:213153154

Change-Id: I4bca6609f0fc2346598a52bd71f2ec60ac7f08f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3653425
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

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