virt-test: fix disable-kvm once more

If config has only 'disable-kvm=yes' but not 'enable-kvm=no'
qemu will be started in kvm mode and with kvm disabled at
compile time it fails to launch.

Currently disable/enable-kvm config options and qemu supported
options are intermingled in a bunch of ifs, separate apples
and oranges in different bins and make logic straightforward:

* alias disable-kvm to enable-kvm param
* get right option set to disable/enable kvm on qemu/qemu-kvm
* set appropriate option depending on value of param enable-kvm

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
virt-test: run cpuid.custom variants without kvm

cpuid.custom will fail if qemu is built without kvm,
due to virt-test tries to start qemu in kvm mode.

To fix it run custom variants in TCG mode, since they do
not have anything kvm specific.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>