snapshot: sanity check when reusing file for snapshot
snapshot: sanity check when reusing file for snapshot

The snapshot code when reusing an existing file had hard-to-read
logic, as well as a missing sanity check: REUSE_EXT should require
the destination to already be present.

* src/qemu/qemu_driver.c (qemuDomainSnapshotDiskPrepare): Require
destination on REUSE_EXT, rename variable for legibility.

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