lib: Don't use -cpu max on RISC-V

qemu (7.0) does not support -cpu max for TCG.

Note this change is necessary but not sufficient for getting
libguestfs to run on RISC-V, because there is also currently no
working path to make -kernel work.

lib/appliance-cpu.c

@@ -84,6 +84,9 @@ guestfs_int_get_cpu_model (int kvm)
 #elif defined(__powerpc64__)
   /* See discussion in https://bugzilla.redhat.com/show_bug.cgi?id=1605071 */
   return NULL;
+#elif defined(__riscv)
+  /* qemu-system-riscv64 (7.0) doesn't yet support -cpu max */
+  return NULL;
 #else
   /* On most architectures we can use "max" to get the best possible CPU.
    * For recent qemu this should work even on TCG.