qemu: Change return type of virQEMUCapsFetchCPUDefinitions

qemu: Change return type of virQEMUCapsFetchCPUDefinitions

The function would return a valid virDomainCapsCPUModelsPtr with empty
CPU models list if query-cpu-definitions exists in QEMU, but returns
GenericError meaning it's not in fact implemented. This behaviour is a
bit strange especially after such virDomainCapsCPUModels structure is
stored in capabilities XML and parsed back, which will result in NULL
virDomainCapsCPUModelsPtr rather than a structure containing nothing.

Let's just keep virDomainCapsCPUModelsPtr NULL if the QMP command is not
implemented and change the return value to int so that callers can
easily check for failure or success.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>