Modifying the xml on either save or restore only gets you so
far - you have to remember to 'virsh dumpxml dom' just prior
to the 'virsh save' in order to have an xml file worth modifying
that won't be rejected due to abi breaks.  To make this more
powerful, we need a way to grab the xml embedded within a state
file, and from there, it's not much harder to also support
modifying a state file in-place.

Also, virDomainGetXMLDesc didn't document its flags.

* include/libvirt/libvirt.h.in (virDomainSaveImageGetXMLDesc)
(virDomainSaveImageDefineXML): New prototypes.
* src/libvirt.c (virDomainSaveImageGetXMLDesc)
(virDomainSaveImageDefineXML): New API.
* src/libvirt_public.syms: Export them.
* src/driver.h (virDrvDomainSaveImageGetXMLDesc)
(virDrvDomainSaveImgeDefineXML): New driver callbacks.