qemu: don't pass virConnectPtr around for secrets

qemu: don't pass virConnectPtr around for secrets

During domain startup there are many places where we need to acquire
secrets. Currently code passes around a virConnectPtr, except in the
places where we pass in NULL. So there are a few codepaths where ability
to start guests using secrets will fail. Change to acquire a handle to
the secret driver when needed.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>