ivshmem-test: Improve test cases /ivshmem/server-*

Document missing test: behavior with MSI-X present but not enabled.

For MSI-X, we test and clear the interrupt pending bit before testing
the interrupt.  For INTx, we only clear.  Change to test and clear for
consistency.

Test MSI-X vector 1 in addition to vector 0.

Improve comments.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-10-git-send-email-armbru@redhat.com>