qemu: Relax validation for mem->access if guest has no NUMA

qemu: Relax validation for mem->access if guest has no NUMA

In v6.8.0-27-g88957116c9 and friends I've switched the way the
default RAM is specified for QEMU (from plain -m to
memory-backend-*). This means, that even if a guest doesn't have
any NUMA nodes configured we can use memory-backend-* attributes
to translate user config requests. For instance, we can allow
memory to be shared (<access mode='shared'/> under
<memoryBacking/>). But what my original commits are missing is
allowing such configuration in our validator.

Fixes: 88957116c9d3cb4705380c3702c9d4315fb500bb
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1839034#c12
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>