ivshmem: Don't update non-existent MSI routes

ivshmem: Don't update non-existent MSI routes

As of commit 660c97eef6f8 ("ivshmem: use kvm irqfd for msi notifications"),
QEMU crashes with:

  kvm_irqchip_commit_routes: Assertion `ret == 0' failed.

if the ivshmem device is configured with more vectors than what the server
supports. This is caused by the ivshmem_vector_unmask() being called on
vectors that have not been initialized by ivshmem_add_kvm_msi_virq().

This commit fixes it by adding a simple check to the mask and unmask
callbacks.

Note that the opposite mismatch, if the server supplies more vectors than
what the device is configured for, is already handled and leads to output
like:

  Too many eventfd received, device has 1 vectors

To reproduce the assert, run:

  ivshmem-server -n 0

and QEMU with:

  -device ivshmem-doorbell,chardev=iv
  -chardev socket,path=/tmp/ivshmem_socket,id=iv

then load the Windows driver, at the time of writing available at:

https://github.com/virtio-win/kvm-guest-drivers-windows/tree/master/ivshmem

The issue is believed to have been masked by other guest drivers, notably
Linux ones, not enabling MSI-X on the device.

Fixes: 660c97eef6f8 ("ivshmem: use kvm irqfd for msi notifications")
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20171211072110.9058-2-lprosek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>