KVM: x86: clean/fix memory barriers in irqchip_in_kernel

The memory barriers are trying to protect against concurrent RCU-based
interrupt injection, but the IRQ routing table is not valid at the time
kvm->arch.vpic is written.  Fix this by writing kvm->arch.vpic last.
kvm_destroy_pic then need not set kvm->arch.vpic to NULL; modify it
to take a struct kvm_pic* and reuse it if the IOAPIC creation fails.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>