qemu: migration: Skip cache=none check for disks which are storage-migrated
qemu: migration: Skip cache=none check for disks which are storage-migrated

Since the disks are copied by qemu, there's no need to enforce
cache=none. Thankfully the code that added qemuMigrateDisk did not break
existing configs, since if you don't select any disk to migrate
explicitly the code behaves sanely.

The logic for determining whether a disk should be migrated is
open-coded since using qemuMigrateDisk twice would be semantically
incorrect.

nertpinx/libvirt,datto/libvirt,jardasgit/libvirt,nertpinx/libvirt,jfehlig/libvirt,eskultety/libvirt,jfehlig/libvirt,crobinso/libvirt,crobinso/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,jardasgit/libvirt,libvirt/libvirt,datto/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,zippy2/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jardasgit/libvirt,olafhering/libvirt,nertpinx/libvirt,datto/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,crobinso/libvirt,andreabolognani/libvirt,crobinso/libvirt,eskultety/libvirt,VenkatDatta/libvirt,zippy2/libvirt,andreabolognani/libvirt,eskultety/libvirt,andreabolognani/libvirt,jfehlig/libvirt,nertpinx/libvirt,olafhering/libvirt,VenkatDatta/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,datto/libvirt,zippy2/libvirt,jardasgit/libvirt,andreabolognani/libvirt,olafhering/libvirt,libvirt/libvirt,eskultety/libvirt,eskultety/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,datto/libvirt