qemu: Fix CPU model fallback in domain capabilities
Our documentation of the domain capabilities XML says that the fallback
attribute of a CPU model is used to indicate whether the CPU model was
detected by libvirt itself (fallback="allow") or by asking the
hypervisor (fallback="forbid"). We need to properly set
fallback="forbid" when CPU model comes from QEMU to match the
documentation.
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>