Commit c12e3580 (Handle libvirt/cpu_model_extra_flags option)
added support for handling invididual CPU feature flags, but
restricted the options only to "PCID".
In this change, we lift the restriction of choices, and allow
to specify multiple CPU feature flags for all three CPU modes
for the libvirt driver: 'custom', 'host-model' and 'host-passthrough'.
NOTE: The related changes are done in change:
Id4f2cad2e0b0c34a7a68d0b3a6f69b44816f4cd9
Change-Id: I688436ca4449e1865549acc7481ef1978644c528
a1ee3a084c