Use proper term in TCG README

In TCG, "target" means the host architecture for which TCG generates
the code. Using "guest" rather than "target" to make the document more
consistent.

Signed-off-by: Chen Wei-Ren <chenwj@iis.sinica.edu.tw>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tcg/README

@@ -14,6 +14,10 @@ the emulated architecture. As TCG started as a generic C backend used
 for cross compiling, it is assumed that the TCG target is different
 from the host, although it is never the case for QEMU.
 
+In this document, we use "guest" to specify what architecture we are
+emulating; "target" always means the TCG target, the machine on which
+we are running QEMU.
+
 A TCG "function" corresponds to a QEMU Translated Block (TB).
 
 A TCG "temporary" is a variable only live in a basic
@@ -379,7 +383,7 @@ double-word product T0.  The later is returned in two single-word outputs.
 
 Similar to mulu2, except the two inputs T1 and T2 are signed.
 
-********* 64-bit target on 32-bit host support
+********* 64-bit guest on 32-bit host support
 
 The following opcodes are internal to TCG.  Thus they are to be implemented by
 32-bit host code generators, but are not to be emitted by guest translators.
@@ -521,9 +525,9 @@ register.
   a better generated code, but it reduces the memory usage of TCG and
   the speed of the translation.
 
-- Don't hesitate to use helpers for complicated or seldom used target
+- Don't hesitate to use helpers for complicated or seldom used guest
   instructions. There is little performance advantage in using TCG to
-  implement target instructions taking more than about twenty TCG
+  implement guest instructions taking more than about twenty TCG
   instructions. Note that this rule of thumb is more applicable to
   helpers doing complex logic or arithmetic, where the C compiler has
   scope to do a good job of optimisation; it is less relevant where
@@ -531,9 +535,9 @@ register.
   inline TCG may still be faster for longer sequences.
 
 - The hard limit on the number of TCG instructions you can generate
-  per target instruction is set by MAX_OP_PER_INSTR in exec-all.h --
+  per guest instruction is set by MAX_OP_PER_INSTR in exec-all.h --
   you cannot exceed this without risking a buffer overrun.
 
 - Use the 'discard' instruction if you know that TCG won't be able to
   prove that a given global is "dead" at a given program point. The
-  x86 target uses it to improve the condition codes optimisation.
+  x86 guest uses it to improve the condition codes optimisation.