lib: Preserve error around virDomainNetReleaseActualDevice()
This function is calling public API virNetworkLookupByName()
which resets the error. Therefore, if
virDomainNetReleaseActualDevice() is used in cleanup path it
actually resets the original error that got us jump into
'cleanup' label.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>