qemu: allow PMU feature to be enabled for ppc64 guests

qemu: allow PMU feature to be enabled for ppc64 guests

The PMU feature is enabled by default in ppc64 guests and can't
be disabled via Libvirt or QEMU [1]. The current PMU feature
implementation does not allow PMU to enabled or disabled in the
ppc64 guest. Declaring the PMU feature will make the 'pmu'
property to be passed on to QEMU, but this property isn't
available for ppc64:

qemu-kvm: can't apply global host-powerpc64-cpu.pmu=on: Property '.pmu' not found

A similar error is thrown when trying to disable the PMU.

This patch standardizes the PMU handling for ppc64 guests by:

- throwing an error if the user attempts to set the feature to
'off', given that this feature can't be turned off at all;

- allowing the feature to be declared as 'on' in the domain XML.
This is done by skipping ppc64 guests when creating the command
line for this feature.

[1] https://www.redhat.com/archives/libvir-list/2020-March/msg00874.html

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>