event: ignore attempts to replace the event loop impl

event: ignore attempts to replace the event loop impl

Although not previously explicitly documented, the expectation for
the libvirt event loop is that an implementation is registered early
in application startup, before calling any libvirt APIs and then
run forever after. Replacing a previously registered event loop is
not safe & subject to races even if virConnectClose has been called
on open handles, due to delayed deregistration of callbacks during
conenction close.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>