virt/kvm/assigned-dev.c: Set 'dev->irq_source_id' to '-1' after free it

As a generic function, deassign_guest_irq() assumes it can be called
even if assign_guest_irq() is not be called successfully (which can be
triggered by ioctl from user mode, indirectly).

So for assign_guest_irq() failure process, need set 'dev->irq_source_id'
to -1 after free 'dev->irq_source_id', or deassign_guest_irq() may free
it again.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>