Use multiple PVRs per CPU model to reduce the number of models we
need to keep track of.

Remove specific CPU models (eg. POWER7+_v2.1): the corresponding
generic CPU model (eg. POWER7) should be used instead to ensure
the guest can be booted on any compatible host.

Get rid of all the entries that did not match any of the CPU
models supported by QEMU, like power8 and power8e.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1250977