remote: refactor how unprivileged user session connection is identified

remote: refactor how unprivileged user session connection is identified

Currently the VIR_DRV_OPEN_REMOTE_USER flag is only set when we identify
that we're connecting to a local libvirtd daemon. We would like to be
able to set that even if connecting to a remote libvirtd daemon. This
entails refactoring the conditional check.

One subtle change is that the VIR_DRV_OPEN_REMOTE_USER is now set when
the test+XXX://  URI is used, even if a servername is present. This has
no effect in this patch, but will later.

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