Goal

• As a user, I want to enable live migration of SCSI Persistent Reservations on an existing libvirt domain (an existing versioned machine type that does not automatically enable it).

Implementation notes{}

• QEMU's scsi-block device is getting a new `migrate-pr=on|off` property that controls whether the live migration stream will include persistent reservation state. When it is off, persistent reservations are not migrated and older QEMU versions can receive the incoming migration successfully. When it is on, persistent reservations are migrated and older QEMU versions fail migration. Note that if the guest currently has no reservation key registered, then migration to an old QEMU will succeed even when `migrate-pr=on` (but we usually cannot rely on this because the guest might start using persistent reservations at any point).
• Upstream QEMU 10.3+ machine types will default to `migrate-pr=on`. Earlier machine types default to `migrate-pr=off` for migration compatibility.

Acceptance criteria

• There is a libvirt domain XML syntax to enable and disable live migration of SCSI persistent reservations.
• Domains configured with the feature enabled can successfully live migrate with SCSI persistent reservations, even when using an older machine type so long as the `migrate-pr=on` option is explicitly enabled.