The new virtio input devices are not that new any more. Support was merged in qemu 2.4 (host) and linux kernel 4.1 (guest). Which means that most distributions should have picked up support for virtio-input meanwhile. libvirt gained support for libvirt-input too (version 1.3.0 & newer), so using virtio-input devices is as simple as adding
<input type='tablet' bus='virtio'/>
to your domain xml configuration. Or replacing the usb tablet with a virtio tablet, possibly eliminating the need to have a usb host adapter in your virtual machine as often the usb tablet is the only usb device.
There are also virtio keyboard and mouse devices. Using them on x86 isn’t very useful as every virtual machine has ps/2 keyboard and mouse anyway. For ppc64, arm and aarch64 architectures the virtio keyboard is a possible alternative to the usb keyboard:
<input type='keyboard' bus='virtio'/>
At the moment the firmware (edk2/slof) lacks support for virtio keyboards, so switching from usb to virtio looses the ability to do any keyboard input before the linux kernel driver loads, i.e. you can’t operate the grub boot menu. I hope this changes in the future.
If you have to stick to the usb keyboard or usb tablet due to missing guest drivers for virtio input I strongly suggest to use xhci as usb host adapter:
<controller type='usb' model='nec-xhci'/> <input type='tablet' bus='usb'/>
xhci emulation needs noticable fewer cpu cycles when compared to uhci, ohci and ehci host adapters. That of course requires xhci driver support in the guest, but that should be less of an issue these days. Windows 7 is probably the only guest without xhci support which is still in widespread use.