runqemu: keep generating tap devices

in case there is no tap device the script tries to
generate a new one.
The new device is then unguarded for a moment, so
the newly generated device could be acquired
by a different instance or user, before it is locked to
the instance with acquire_taplock.
To fix that keep generating new tap devices in case
the lock can't be acquired up to 5 times.
If no tap device can be locked it fails in the existing
error handling

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>