Fix startup of LXC containers with filesystems containing symlinks

Fix startup of LXC containers with filesystems containing symlinks

Given an LXC guest with a root filesystem path of

  /export/lxc/roots/helloworld/root

During startup, we will pivot the root filesystem to end up
at

  /.oldroot/export/lxc/roots/helloworld/root

We then try to open

  /.oldroot/export/lxc/roots/helloworld/root/dev/pts

Now consider if '/export/lxc' is an absolute symlink pointing
to '/media/lxc'. The kernel will try to open

  /media/lxc/roots/helloworld/root/dev/pts

whereas it should be trying to open

  /.oldroot//media/lxc/roots/helloworld/root/dev/pts

To deal with the fact that the root filesystem can be moved,
we need to resolve symlinks in *any* part of the filesystem
source path.

* src/libvirt_private.syms, src/util/util.c,
  src/util/util.h: Add virFileResolveAllLinks to resolve
  all symlinks in a path
* src/lxc/lxc_container.c: Resolve all symlinks in filesystem
  paths during startup