lxc: be more patient while resolving symlinks

lxc: be more patient while resolving symlinks

Resolving symlinks can fail before mounting any file system if one file
system depends on another being mounted. Symlinks are now resolved in
two passes:

  * Before any file system is mounted, but then we are more gentle if
    the source path can't be accessed
  * Right before mounting a file system, so that we are sure that we
    have the resolved path... but then if it can't be accessed we raise
    an error.