accel: Make AccelClass.available() optional

When we move accel classes outside accel.c, the available() function
won't be necessary anymore, because the classes will be registered only
if the accelerator code is really enabled at build time.
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

accel.c

@@ -82,7 +82,7 @@ int configure_accelerator(MachineClass *mc)
             fprintf(stderr, "\"%s\" accelerator does not exist.\n", buf);
             continue;
         }
-        if (!acc->available()) {
+        if (acc->available && !acc->available()) {
             printf("%s not supported for this target\n",
                    acc->name);
             continue;