selftests: kvm: report failed stage when exit reason is unexpected

When we get a report like

==== Test Assertion Failure ====
  x86_64/state_test.c:157: run->exit_reason == KVM_EXIT_IO
  pid=955 tid=955 - Success
     1	0x0000000000401350: main at state_test.c:154
     2	0x00007fc31c9e9412: ?? ??:0
     3	0x000000000040159d: _start at ??:?
  Unexpected exit reason: 8 (SHUTDOWN),

it is not obvious which particular stage failed. Add the info.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>