Taking or reverting an internal snapshot redefined a pflash VM with a
rom loader around the operation. libvirt 10.10 rejects that definition,
so the request ended in a 500; where it was accepted, the snapshot
recorded the fake loader, and a failure between rewrite and restore
left the VM unable to boot.
The definition is no longer touched: libvirt decides, and its error is
shown (for a running VM with raw NVRAM: 'internal snapshots of a VM
with pflash based firmware require QCOW2 nvram format'). A stopped VM's
disk-only internal snapshot is taken, as libvirt allows it. Revert
errors are shown too. The external snapshot path keeps its rewrite for
now.
Live tests cover running and stopped UEFI VMs; both failed with a 500
on the previous code.
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