syncproc: Add blocking mode.

By setting `proc.blocking = True` it will make reading and writing move
to blocking mode.

This is useful when you don't want read operations to return when no
data is available but instead wait for it.

Change-Id: Id406249bfd63f59f273d27614a38a43595f1d7e1
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21897
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>