qemuBuildNumaArgStr: Use modern -numa memdev= if old -numa mem= is unsupported

qemuBuildNumaArgStr: Use modern -numa memdev= if old -numa mem= is unsupported

In previous commit we started tracking whether QEMU supports
'-numa mem='. This is tied to the machine type because migration
from '-numa mem=' to '-numa memdev' is impossible (or vice
versa). But since it's tied to a machine type (where migration
from one to another is also unsupported) we can allow QEMU to get
rid of the deprecated command line.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1783355
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>