execdriver: Make GetPidsForContainer() a driver call

The current implementation is lxc specific.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Add pkg/rpcfd

This is a small unix-domain-sockets only wrapper around net/rpc
that adds support for passing file descriptors in rpc messages.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Add pkg/libvirt

This is a small wrapper around libvirt, which will be used by the
libvirt exec driver. This is extracted for two reasons:

1) Move all the C-specific types and unsafe use out of the driver.

2) Having this in a separate submodule means the dependency on the
   libvirt library will only be pulled into the main docker binary,
   not dockerinit. This is not possible if the 'import "C"' code
   is part of the libvirt exec driver, *even* if that file is excluded
   from the build.

This code is in mostly based on the previous work by Josh Poimboe.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Add .dockersocket directory to init layer

This will be used by the libvirt exec driver to mount in a directory
with the rpc socket, allowing the daemon to interact with the dockerinit

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Initial version of libvirt execdriver

This is a work in progress, it works at least for trivial cases, but
doesn't support networking yet, and will need more work.

Also, the backend is only build for the dynamic binary, because of problems
linking statically to libvirt.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)