https://bugzilla.redhat.com/show_bug.cgi?id=1534418

Just like ec982f6d denies hugepages for non-existent
guest NUMA nodes in case there are some nodes configured.
Unfortunately, when there are none, qemuBuildNumaArgStr() is not
called and thus we have to have check in qemuBuildMemPathStr()
too.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>