qemu_migration: Store original migration params in status XML

qemu_migration: Store original migration params in status XML

We keep original values of migration parameters so that we can restore
them at the end of migration to make sure later migration does not use
some random values. However, this does not really work when libvirt
daemon is restarted on the source host because we failed to explicitly
save the status XML after getting the migration parameters from QEMU.
Actually it might work if the status XML is written later for some other
reason such as domain state change, but that's not how it should work.

https://bugzilla.redhat.com/show_bug.cgi?id=2107892

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>