https://bugzilla.redhat.com/show_bug.cgi?id=1710575

It may happen that the system where libvirt is built at doesn't
have udevadm binary but the one where it runs does have it.
If we change how udevadm is run in virWaitForDevices() then we
can safely pass a default value in m4 macro.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>