Prior to this patch, an attempt to register an event without an
event loop started results in the vague:

libvirt: Remote Driver error : adding cb to list

Now it gives the much nicer:

libvirt:  error : internal error: could not initialize domain event timer

This also avoids hiding other reasonable error messages, such as
attempts to register a duplicate callback or OOM errors.

* src/remote/remote_driver.c (remoteConnectNetworkEventRegisterAny)
(remoteConnectDomainEventRegister)
(remoteConnectDomainEventRegisterAny): Preserve more detailed error.
Signed-off-by: NEric Blake <eblake@redhat.com>