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 <peng.hao2@zte.com.cn>
Reviewed-by: Wang Yechao <wang.yechao255@zte.com.cn>