KVM: x86/i8259: Remove a dead store of irq in a conditional block

The [clang-analyzer-deadcode.DeadStores] helper reports
that the value stored to 'irq' is never read.

Signed-off-by: Like Xu <likexu@tencent.com>
Message-Id: <20220301120217.38092-1-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>