libxl: Use consistent error preservation and restoration calls

libxl: Use consistent error preservation and restoration calls

Provide some consistency over error message variable name and usage
when saving error messages across possible other errors or possibility
of resetting of the last error.

Instead of virSaveLastError paired up with virSetError and virFreeError,
we should use the newer virErrorPreserveLast and virRestoreError.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>