kvm tools: Set active console before running /bin/sh

This patch sets active console to the serial console before running the
shell. Doing so fixes two issues:

* Fix job control.

* Set all env vars.

The user visible issues are less warnings (no more of this:
  sh: cannot set terminal process group (-1): Inappropriate ioctl for device
  sh: no job control in this shell)

A working 'top', and a working ctrl-c.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>