run: Check err for loading config file.

In case the instance directory is present but the instance config file is
absent, we will dereference nil pointer.

E.g.

   $ ls -l /Users/asias/.capstan/instances/vbox/node-express-example
   total 0
   srwxr-xr-x  1 asias  staff  0 Nov 19 08:09 node-express-example.sock

Fix by checking err returned by LoadConfig().

Signed-off-by: Asias He <asias@cloudius-systems.com>
vbox: Delete sock file

VBox does not delete the sock file anymore using

  VBoxManage("unregistervm", name, "--delete")

Delete it ourself.
Merge pull request #131 from asias/vbox

Vbox
build: Make capstan version work

$ capstan build capstan.go

will generate a binary named capstan, so rename from
capstan/capstan/*.go to capstan/util/*.go to fix the conflict.

go install does not work well with the

   -ldflags "-X main.VERSION '$version'"

trick. Fix by using go build instead.

Signed-off-by: Asias He <asias@cloudius-systems.com>