supervdsmServer: Cleaner way to wait for signals

supervdsmServer was using silly way to wait for termination, working
around Python bug. This patch use the standard way to wait until
termination signal is received, eliminating the workaround.

Change-Id: I7f0a1fdd518df1524441fd856fc764f8dbd7cf79
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22719
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>