qemu_capabilities: Introduce virQEMUCapsGetCPUFeatures
    
    The function creates a list of all (or migratable only) CPU features
    supported by QEMU. It works by looking at the CPU model info returned by
    query-cpu-model-expansion QMP command.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Ján Tomko <jtomko@redhat.com>
    Reviewed-by: Collin Walling <walling@linux.ibm.com>