volume: Fix exception re-rasing

When re-raising and excepition, the correct way is:

    raise

And not:

    raise e

The later form create a new useless traceback, strating at the point were the
exception was re-raised:

    Traceback (most recent call last):
      File "raise.py", line 14, in <module>
        raise e
    Exception: testing

Instead of the original traceback, showing the original call stack:

    Traceback (most recent call last):
      File "raise.py", line 11, in <module>
        a()
      File "raise.py", line 2, in a
        b()
      File "raise.py", line 5, in b
        c()
      File "raise.py", line 8, in c
        raise Exception("testing")
    Exception: testing

Change-Id: I3f505f4ff70c528b5decd2665f9cd983eb58f605
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33556
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>