ivshmem-test: Clean up wait for devices to become operational

test_ivshmem_server() waits until the first byte in BAR 2 contains the
0x42 we put into shared memory.  Works because the byte reads zero
until the device maps the shared memory gotten from the server.

Check the IVPosition register instead: it's initially -1, and becomes
non-negative right when the device maps the share memory, so no
change, just cleaner, because it's what guest software is supposed to
do.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-9-git-send-email-armbru@redhat.com>