move vm_start to cpus.c

This patch:

* moves vm_start to cpus.c.
* exports qemu_vmstop_requested, since it's needed by vm_start.
* extracts vm_prepare_start from vm_start; it does what vm_start did,
  except restarting the cpus.
* vm_start now calls vm_prepare_start and then restarts the cpus.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Message-Id: <1487092068-16562-2-git-send-email-imbrenda@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>