exec: Save CPUState::exception_index field

This patch adds a subsection with exception_index field to the VMState for
correct saving the CPU state.
Without this patch, simulator could miss the pending exception in the saved
virtual machine state.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Signed-off-by: Andreas Färber <afaerber@suse.de>