cpu: Don't clear cpu->exit_request on reset

cpu->exit_request is part of the execution environment and should
not be cleared when a CPU resets.

Otherwise, we might deadlock QEMU if a CPU resets while there is
I/O going on.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>