virDomainMemoryDefValidate: Check for overlapping memory devices

As of v9.4.0-rc2~5 it is possible to specify guest address where
a virtio-mem/virtio-pmem memory device is mapped to. What that
commit forgot to introduce was a check for overlaps.

And yes, this is technically an O(n^2) algorithm, as
virDomainMemoryDefValidate() is called over each memory device
and after this, virDomainMemoryDefValidate() also iterates over
each memory device. But given there's usually only a handful of
such devices, and this runs only when parsing domain XML I guess
code readability wins over some less obvious solution.

Resolves: https://issues.redhat.com/browse/RHEL-4452


Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>