Currently the only use of the periodic timer tick in kvmtool is to
handle reading from stdin. Though functional, this periodic tick can be
problematic on slow (eg FPGA) platforms and can cause low interactivity or
even stop the execution from progressing at all.

This patch removes the periodic tick in favour of a dedicated thread blocked
waiting for input from the console. In order to reflect the new behaviour,
the old 'kvm__arch_periodic_tick' function is renamed to 'kvm__arch_read_term'.

In making this change it is necessary to actively flush the emulated serial
console's output buffer after the guest writes to it, as otherwise flushing
only happens with terminal input. Similarly, it is no longer necessary to
flush the buffer when we process input.

Signed-off-by: Jonathan Austin <jonathan.austin@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>