qemu: Use memory-backing-file only when needed
    
    We are using memory-backing-file even when it's not needed, for example
    if user requests hugepages for memory backing, but does not specify any
    pagesize or memory node pinning.  This causes migrations to fail when
    migrating from older libvirt that did not do this.  So similarly to
    commit 7832fac84741d65e851dbdbfaf474785cbfdcf3c which does it for
    memory-backend-ram, this commit makes is more generic and
    backend-agnostic, so the backend is not used if there is no specific
    pagesize of hugepages requested, no nodeset the memory node should be
    bound to, no memory access change required, and so on.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1266856
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>