Double allocated varnishlog buffer space.

Extensive test cases often lead to a lot of log output,
which at times tripped the following assert:

Assert error in vtc_log_emit(), vtc_log.c line 122:
  Condition(vtclog_left > l) not true.

Double the amount of buffer space allowed to avoid this. Another 256KB
memory use per concurrent running test should be fine.
Merge branch 'master' of git.varnish-cache.org:varnish-cache