Commit 690e29b9 ("vfio-ccw: Refactor ccw irq handler") changed
one of the checks for the IRQ notifier registration from saying
"the host needs to recognize the only IRQ that exists" to saying
"the host needs to recognize ANY IRQ that exists."

And this worked fine, because the subsequent change to support the
CRW IRQ notifier doesn't get into this code when running on an older
kernel, thanks to a guard by a capability region. The later addition
of the REQ(uest) IRQ by commit b2f96f9e ("vfio-ccw: Connect the
device request notifier") broke this assumption because there is no
matching capability region. Thus, running new QEMU on an older
kernel fails with:

  vfio: unexpected number of irqs 2

Let's adapt the message here so that there's a better clue of what
IRQ is missing.

Furthermore, let's make the REQ(uest) IRQ not fail when attempting
to register it, to permit running vfio-ccw on a newer QEMU with an
older kernel.

Fixes: b2f96f9e ("vfio-ccw: Connect the device request notifier")
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Message-Id: <20210421152053.2379873-1-farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>