[guest] Move garnet/bin/guest to src/virtualization.

Rearrange files into a new structure under "src/virtualization", as
follows:

  garnet/bin/guest/MAINTAINERS  🠞 src/virtualization/MAINTAINERS
  garnet/bin/guest/README.md    🠞 src/virtualization/docs/README.md
  garnet/bin/guest/integration  🠞 src/virtualization/tests
  garnet/bin/guest/cli          🠞 src/virtualization/bin/guest
  garnet/bin/guest/manager      🠞 src/virtualization/bin/guest_manager
  garnet/bin/guest/pkg          🠞 src/virtualization/packages
  garnet/bin/guest/runner       🠞 src/virtualization/bin/guest_runner
  garnet/bin/guest/scripts      🠞 src/virtualization/scripts
  garnet/bin/guest/third_party  🠞 src/virtualization/third_party
  garnet/bin/guest/vmm          🠞 src/virtualization/bin/vmm

The vast majority of changes are simply moves; updating paths in
sources files with:

  sed -e 's/$old_path/$new_path/g'

and manual changes to BUILD.gn files to reflect to new directory
structure.

There are plans to split out the various virtio devices into their own
directories under src/virtualization/bin; this doesn't take place in
this patch.

Change-Id: Ic61c82ecef5880ff95d6ba77064253dc9d7a313f