cpu-exec: avoid repeated sigsetjmp on interrupts

The sigsetjmp only needs to be prepared once for the whole execution
of cpu_exec.  This patch takes care of the "== 0" side, using a
nested loop so that cpu_handle_interrupt goes straight back to
cpu_handle_exception without doing another sigsetjmp.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
