qemu: report error if missing model name when baselining

qemu: report error if missing model name when baselining

When executing the hypervisor-cpu-baseline command and the
XML file contains a CPU definition without a model name, or
an invalid CPU definition, then the commands will fail and
return an error message from the QMP response.

Let's clean this up by checking for a valid definition and
presence of a model name.

This code is copied from virCPUBaseline.

Signed-off-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>