py: add wrappers for libct_container_spawn_execv*fds

Add wrappers for functions libct_container_spawn_execv*fds.
You should use Container.spawn_execvfds and
Container.spawn_execvefds:

ct.spawn_execvfds("/bin/bash", ['/bin/bash'], [0, 1, 2])

Signed-off-by: Dmitry Guryanov <dguryanov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>