message: [qa] Allow passing prev as hexstr in create_block
  Converts hex string to int for you. Complain if nTime is not an int.
diff: @@ -17,7 +17,11 @@ def create_block(hashprev, coinbase, nTime=None, txns=None, ctor=True):
  import time
  block.nTime = int(time.time()+600)
  else:
  + if type(nTime) is not int:
  + raise ValueError("nTime should be int, got {}".format(type(nTime)))
  block.nTime = nTime
  + if type(hashprev) is str:
  + hashprev = int(hashprev, 16)
  block.hashPrevBlock = hashprev
  block.nBits = 0x207fffff # Will break after a difficulty adjustment...
  if coinbase:
  