kvm-vfio: Decouple only when we match a group

Unset-KVM and decrement-assignment only when we find the group in our
list.  Otherwise we can get out of sync if the user triggers this for
groups that aren't currently on our list.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org