vdsm: Shuting down protocol detector fails

When protocol detector is stopped during shutdown, an exception was
raised. The stop code ignores EPIPE which means that the other side of
the pipe is disconnected.

Bug-Url: https://bugzilla.redhat.com/1115104
Change-Id: I82119a61835fe335f2aa5da29fb8d3f2b8ae33fc
Reviewed-on: http://gerrit.ovirt.org/29556
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/30030
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>