qemuDomainAttachDeviceMknodHelper: Create more files in a single go

qemuDomainAttachDeviceMknodHelper: Create more files in a single go

So far, when attaching a device needs two or more /dev nodes
created into a domain, we fork off and run the helper for every
node separately. For majority of devices this is okay, because
they need no or one node created anyway. But the idea is to use
this attach code to build the namespace when starting a domain,
in which case there will be way more nodes than one.

To achieve this, the recursive approach for handling symlinks has
to be turned into an iterative one.

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