Moved some functions from lxc driver to execdriver utils

Some of the internal functions of the LXC driver would be needed for a
Libvirt driver as those two have quite some similar constraints. Moving
them to execdriver/utils.go to help code sharing.

Signed-off-by: Cédric Bosdonnat <cedric.bosdonnat@free.fr>
add libvirt execdriver

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Signed-off-by: Cédric Bosdonnat <cedric.bosdonnat@free.fr>
Add dockerinit tag to dynbinary builds

This excludes the libvirt execdriver go files depending on the libvirt
shared library when building dockerinit. Without this, ld can't find
-lvirt when building dockerinit... which will happen for all distros
not shipping the libvirt.a file.

Signed-off-by: Cédric Bosdonnat <cedric.bosdonnat@free.fr>
libvirt driver: implement Pause and Unpause

Signed-off-by: Cédric Bosdonnat <cedric.bosdonnat@free.fr>
libvirt driver: create Terminal objects

Without the Terminal object, how can the console be resized?

Signed-off-by: Cédric Bosdonnat <cedric.bosdonnat@free.fr>
libvirt: the rpc socket file path is now random and secure

Signed-off-by: Cédric Bosdonnat <cedric.bosdonnat@free.fr>