runqemu: Fix automated call to runqemu-ifup

runqemu: Fix automated call to runqemu-ifup

When runqemu tries to call runqemu-ifup to create tap devices, it checks the
output of runqemu-ifup to get the newly created tap device.

The behavior of runqemu-ifup was recently modified along with its output, it
no longer expects the uid parameter to be passed and it prints out a warning
if it was, since this warning was now part of the output runqemu tries to parse
it and convert it to an int() which proved impossible.

Pass the correct arguments to the runqemu-ifup call and echo the warning
to stderr instead to make sure its not being parsed and used by runqemu in any
case.

Signed-off-by: Alejandro Enedino Hernandez Samaniego <alejandro@enedino.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>