scripts/runqemu: Improve lockfile handling for python with close_fd=True

scripts/runqemu: Improve lockfile handling for python with close_fd=True

On python versions with close_fds=True (python 3.2 onwards), the tap
device lockfile isn't passed to the child process.

Since this guards against use of an active interface, we really want this
here, so pass it in pass_fds. This means if the parent exits early, the child
still holds the lock, avoiding messages like:

runqemu - ERROR - Failed to run qemu: qemu-system-x86_64: could not configure /dev/net/tun (tap0): Device or resource busy

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>