Document the cpuType default for riscv64

Signed-off-by: Anders F Björklund <anders.f.bjorklund@gmail.com>

Author: afbjorklund

examples/default.yaml

@@ -259,6 +259,8 @@ cpuType:
   armv7l: null
   # 🟢 Builtin default: "qemu64" (or "host,-pdpe1gb" when running on x86_64 host)
   x86_64: null
+  # 🟢 Builtin default: "rv64" (or "host" when running on riscv64 host)
+  riscv64: null
 
 rosetta:
   # Enable Rosetta for Linux (EXPERIMENTAL).