Better KVM detection in autotools

Beside searching for libvirt, check if the kvm module is actually loaded into the kernel. It's still not perfect, but since that's what the kvm FAQ recommends to do, it's a OK (http://www.linux-kvm.org/page/FAQ#How_can_I_check_that_I.27m_not_falling_back_to_QEMU_with_no_hardware_acceleration.3F)