semihosting: Add GuestFDConsole

Add a GuestFDType for connecting to the semihosting console.
Hook up to read, write, isatty, and fstat syscalls.

Note that the arm-specific syscall flen cannot be applied
to the console, because the console is not a descriptor
exposed to the guest.

Reviewed-by: Luc Michel <lmichel@kalray.eu>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/semihosting/guestfd.h

@@ -13,9 +13,10 @@
 
 typedef enum GuestFDType {
     GuestFDUnused = 0,
-    GuestFDHost = 1,
-    GuestFDGDB = 2,
-    GuestFDStatic = 3,
+    GuestFDHost,
+    GuestFDGDB,
+    GuestFDStatic,
+    GuestFDConsole,
 } GuestFDType;
 
 /*