aspeed: Support runqemu

 $ runqemu nographic slirp

We require nographic instead of setting -nographic through QB_OPT_APPEND
in order to avoid the serial port being sent to a character device:

   self.qemu_opt += " -serial mon:vc"

We require slirp instead of setting QB_SLIRP_OPT as we are happy with
the default spirp options, we just want to enable slirp instead of the
other networking options that require root access.

Change-Id: I2ad8a8237ad1a0f5fa888991f7f58a6266a58caf
Signed-off-by: Joel Stanley <joel@jms.id.au>