KVM: x86: drop PIO from unregistered devices

KVM protects the device list with SRCU, and therefore different calls
to kvm_io_bus_read()/kvm_io_bus_write() can very well see different
incarnations of kvm->buses.  If userspace unregisters a device while
vCPUs are running there is no well-defined result.  This patch applies
a safe fallback by returning early from emulator_pio_in_out().  This
corresponds to returning zeroes from IN, and dropping the writes on
the floor for OUT.

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