hsm: Do not hide errors when checking device visibility

When checking devices visibility, all errors from os.stat() are treated
as "device does not exists", hiding fatal error and making it harder to
debug and fix them.

This patch handle only ENOENT for checking visibility and raise other
errors we cannot handle.

Change-Id: I7411d7d2eaa9172f612fd1b1e04177bef270ab19
Relates-To: https://bugzilla.redhat.com/923773
Bug-Url: https://bugzilla.redhat.com/1044068
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21211
Reviewed-by: Ayal Baron <abaron@redhat.com>