qemu: fix netdev alias name assignment wrt type='hostdev'

qemu: fix netdev alias name assignment wrt type='hostdev'

This patch resolves:

   https://bugzilla.redhat.com/show_bug.cgi?id=827519

The problem is that an interface with type='hostdev' will have an
alias of the form "hostdev%d", while the function that looks through
existing netdevs to determine the name to use for a new addition will
fail if there's an existing entry that does not match the form
"net%d".

This is another of the handful of places that need an exception due to
the hybrid nature of <interface type='hostdev'> (which is not exactly
an <interface> or a <hostdev>, but is both at the same time).