qemu: Avoid memory leak in virQEMUCapsCPUDefsToModels

qemu: Avoid memory leak in virQEMUCapsCPUDefsToModels

The @vendor variable inside virQEMUCapsCPUDefsToModels() is
allocated, but never freed. But there is actually no need for it
to be allocated, because it merely passes a retval of
virCPUGetVendorForModel() (which returns a const string) to
virDomainCapsCPUModelsAdd() (which ten accepts the argument as
const string). Therefore, drop the g_strdup() call and fix the
type of the variable.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>