qemu: process: Call disk startup policy check after cloning domain def

qemu: process: Call disk startup policy check after cloning domain def

In commit 1e38ef72 the disk startup policy check was moved prior to the
call to virDomainObjSetDefTransient which dropped the disk from the
config rather than the def to be started which is a bug.

Additionally we'd not report the disk change event for this since the
disk aliases were not set at that point.

Finally 'volume' based disks would not work with startup policy too.

Fix it by moving it back after the definition is copied, aliases are
assigned and disk sources are translated.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1341415