qemu: Use memory-backing-file only when needed

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>