Volume detection in the scsi backend was duplicating code already
present in storage_backend.c. Let's drop the duplicate code.

Also, change the shared function name to be less generic, and remove
some error squashing in the other call site.

We were squashing error messages in a few cases. Recode to follow common
ret = -1 convention.

v2: Handle more error squashing issues further up in MakeNewVol and
    CreateVols. Use ret = -1 convention in MakeVols.

* src/storage/storage_backend_mpath.c (virStorageBackendIsMultipath):
The result of dm_get_next_target was never used (and isn't needed),
so don't store it.

The virConnectPtr is no longer required for error reporting since
that is recorded in a thread local. Remove use of virConnectPtr
from all APIs in storage_conf.{h,c} and storage_encryption_conf.{h,c}
and update all callers to match

It was used for error reporting only.

* daemon/qemud.c, src/Makefile.am: Adapt for changed paths
* src/storage*.c, src/storage/*.h, src/parthelpre.c: Move
  to src/storage/

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

* configure.in src/Makefile.am src/storage_backend.[ch]
  src/storage_conf.[ch] src/storage_backend_mpath.[ch] po/POTFILES.in:
  add a new module for storage multipath, it requires device-mapper