kvm: irqchip: fix memory leak

We were taking the exit path after checking ue->flags and return value
of setup_routing_entry(), but 'e' was not freed incase of a failure.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>