vfio-pci: Don't use kvm_irqchip_in_kernel

kvm_irqchip_in_kernel() has an architecture specific meaning, so
we shouldn't be using it to determine whether to enabled KVM INTx
bypass.  kvm_irqfds_enabled() seems most appropriate.  Also use this
to protect our other call to kvm_check_extension() as that explodes
when KVM isn't enabled.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: qemu-stable@nongnu.org
