Allow sync IO and keepalives to be skipped in RPC client setup

Allow sync IO and keepalives to be skipped in RPC client setup

Currently the virNetClientPtr constructor will always register
the async IO event handler and the keepalive objects. In the
case of the lock manager, there will be no event loop available
nor keepalive support required. Split this setup out of the
constructor and into separate methods.

The remote driver will enable async IO and keepalives, while
the LXC driver will only enable async IO

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>