libxl: Fix domain startup failure error reporting

libxl: Fix domain startup failure error reporting

When domain startup fails, domain cleanup calls
libxlNetworkUnwindDevices, which calls virGetConnectNetwork, which
is a top level API entry point, which resets the initial saved error,
leading to clients seeing:

  error: An error occurred, but the cause is unknown

This preserves the error around the entire teardown process, similar
to what is done in the qemu driver.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>