arm/arm64: KVM : Enable vhost device selection under KVM config menu

vhost drivers provide guest VMs with better I/O performance and lower
CPU utilization. This patch allows users to select vhost devices under
KVM configuration menu on ARM. This makes vhost support on arm/arm64
on a par with other architectures (e.g. x86, ppc).

Signed-off-by: Wei Huang <wei@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>