qemu: shared disks with cache=directsync should be safe for migration
qemu: shared disks with cache=directsync should be safe for migration

At present shared disks can be migrated with either readonly or cache=none. But
cache=directsync should be safe for migration, because both cache=directsync and cache=none
don't use the host page cache, and cache=direct write through qemu block layer cache.

Signed-off-by: Peng Hao <8c0002d22ae9f34a326723c818121ff67611b65b@zte.com.cn>
Reviewed-by: Wang Yechao <d156c2278b59b97b85e8218b42611783bb8435e2@zte.com.cn>

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