exec: Remove cpu from cpus list during cpu_exec_exit()

CPUState *cpu gets added to the cpus list during cpu_exec_init(). It
should be removed from cpu_exec_exit().

cpu_exec_exit() is called from generic CPU::instance_finalize and some
archs like PowerPC call it from CPU unrealizefn. So ensure that we
dequeue the cpu only once.

Now -1 value for cpu->cpu_index indicates that we have already dequeued
the cpu for CONFIG_USER_ONLY case also.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
