readlines - remove redundant pathExists check

fileSD -> readlines:
* The verification of path existence is redundant.
* Try to read the metadata file and handle an exception:
- If no such file: return an empty list.
- Else, raise the exception.

Change-Id: Id73f4bd177465f253eb4f65403b17ff7375e329c
Bug-Url: https://bugzilla.redhat.com/882667
Signed-off-by: Daniel Erez <derez@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13664
Reviewed-by: Dan Kenigsberg <danken@redhat.com>