fixup: don't use exists boolean

fixup: don't use exists boolean

The previous condition didn't fail tests because it was
always false. The 'exists' property is not actually usable
when writing the interface file. It is merely a hint that
the interface existed in /proc/net/dev while parsing the
interfaces file and is otherwise actually unused here.

Simply check for the existence of the interface in $ifaces.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>