Fix use of dlopen modules

Fix use of dlopen modules

Remove the bogus dependancy between node_device.c & storage_backend.c
by moving the virWaitForDevices into util.h where it can be shared
safely

* src/storage_backend_disk.c, src/storage_backend_logical.c,
  src/storage_backend_mpath.c, src/storage_backend_scsi.c: Replace
  virStorageBackendWaitForDevices with virFileWaitForDevices
* src/storage_backend.c, src/storage_backend.h: Remove
  virStorageBackendWaitForDevices, virWaitForDevices
* src/util.h, src/util.c: Add virFileWaitForDevices
* configure.in: Move xmlrpc check further down after pkgconfig
  is detected
* src/Makefile.am: Add missing XMLRPC_CFLAGS/LIBS to opennebula
* src/libvirt_private.syms: Add many missing exports