Cloning a UEFI VM defined a persistent storage pool on the NVRAM's
directory when no pool held it, and dropped <nvram> from the clone,
which found its copy only because libvirt chose the same file name.
The copy is now made through the NVRAM directory's pool (a transient one
when there is none) or the pool that holds the file, with the original's
format, owner and mode, and the clone's definition names it. A failed
definition removes it with the copied disks. An NVRAM that is not a file
is refused before anything is copied.
