Do not try to create physical interfaces

Do not try to create physical interfaces

vifscreate() always creates all virtual interfaces up-front.

To check whether a given interface exists, ifstart() uses ifcreate()
which tries to create nonexistent ones.

Virtual ones are guaranteed to be present and physical ones cannot be
created, so replace the ifcreate() call with a simpler ifconfig test and
clarify the comment.

OK martijn afresh1