save: support qemu modifying xml on domain save/restore

save: support qemu modifying xml on domain save/restore

With this, it is possible to update the path to a disk backing
image on either the save or restore action, without having to
binary edit the XML embedded in the state file.

This also modifies virDomainSave to output a smaller xml (only
the inactive xml, which is all the more virDomainRestore parses),
while still guaranteeing padding for most typical abi-compatible
xml replacements, necessary so that the next patch for
virDomainSaveImageDefineXML will not cause unnecessary
modifications to the save image file.

* src/qemu/qemu_driver.c (qemuDomainSaveInternal): Add parameter,
only use inactive state, and guarantee padding.
(qemuDomainSaveImageOpen): Add parameter.
(qemuDomainSaveFlags, qemuDomainManagedSave)
(qemuDomainRestoreFlags, qemuDomainObjRestore): Update callers.