poky-qemu-internal: Support use of a preconfigured tap device
This patch makes poky-qemu-internal check for the existence of an
available preconfigured tap device before running poky-qemu-ifup to
make a new one.
Locking is handled with a lockfile in /tmp/qemu-tap-locks/. This uses
the lockfile utility, so that needs to be present on the host. On
exit, this script removes the lock file so that the tap device may be
reused.