ivshmem-test: Improve test case /ivshmem/single

Test state of registers after reset.

Test reading Interrupt Status clears it.

Test (invalid) read of Doorbell.

Add more comments.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-8-git-send-email-armbru@redhat.com>
