KVM: x86: inline kernel_pio into its sole caller

The caller of kernel_pio already has arguments for most of what kernel_pio
fishes out of vcpu->arch.pio.  This is the first step towards ensuring that
vcpu->arch.pio.* is only used when exiting to userspace.

No functional change intended.

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