Bug description: QEMU currently gets all bits from GET_SUPPORTED_CPUID
for CPUID leaf 0xA and passes them directly to the guest. This makes
the guest ABI depend on host kernel and host CPU capabilities, and
breaks live migration if we migrate between hosts with different
capabilities (e.g., different number of PMU counters).

Add a "pmu" property to X86CPU, and set it to true only on "-cpu host",
or on pc-*-1.5 and older machine-types.

For now, setting pmu=on will enable the current passthrough mode that
doesn't have any ABI stability guarantees, but in the future we may
implement a mode where the PMU CPUID bits are stable and configurable.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>