runqemu: handle tap device creation failure properly

runqemu: handle tap device creation failure properly

If we fail to run the command to generate the tap devices then we should
show a reasonable message and then exit, without showing a traceback.
"return 1" at this point in the code does nothing because the caller
doesn't check the return, so just use sys.exit().

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>