make libvirt optional

[chungers]

This PR makes the inclusion of libvirt in the core binary optional.

To build with libvirt enabled:

GO_BUILD_TAGS=libvirt make build/infrakit

This will include libvirt. libvirt go binding used here isn't a pure Go binding so many users have run into problems with building locally. Cross compilation and linking statically of infrakit with libvirt will be fixed in a future PR. Currently there are some problems with -extldflags '-static' when cross compiling libvirt inside a container.

Signed-off-by: David Chung david.chung@docker.com

[codecov]

Codecov Report

Merging #718 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #718      +/-   ##
=========================================
+ Coverage   46.54%   46.6%   +0.06%     
=========================================
  Files          85      85              
  Lines        9417    9417              
=========================================
+ Hits         4383    4389       +6     
+ Misses       4700    4695       -5     
+ Partials      334     333       -1

Impacted Files	Coverage Δ
pkg/provider/google/plugin/gcloud/tags.go	82.35% <0%> (+2.94%)	⬆️
pkg/rpc/mux/server.go	47.5% <0%> (+4.16%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0755e99...3ccb1cf. Read the comment docs.