fix error handling in isdir and islink

isdir and islink implemented using ioprocess will
throw an exception in case of stat failure because of
errno ENOENT, while it should return False.

Change-Id: Iaa7d1a16ba2940963d29a19a7140235fbd80ba8d
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1115123
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29633
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>