plugins: extend execlog to filter matches

Sometimes the whole execlog is just two much so add the ability to
filter by instruction opcode or address.

[AJB: this shows for example

 qemu-system-aarch64 -display none -serial mon:stdio \
   -M virt -cpu max \
   -semihosting-config enable=on \
   -kernel ./tests/tcg/aarch64-softmmu/memory-sve \
   -plugin ./contrib/plugins/libexeclog.so,ifilter=st1w,afilter=0x40001808 -d plugin -D plugin.out

the st1w SVE instruction is not instrumenting its stores.]

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alexandre Iooss <erdnaxe@crans.org>
Cc: Robert Henry <robhenry@microsoft.com>
Cc: Aaron Lindsay <aaron@os.amperecomputing.com>
Message-Id: <20220929114231.583801-36-alex.bennee@linaro.org>

docs/devel/tcg-plugins.rst

@@ -346,7 +346,7 @@ The execlog tool traces executed instructions with memory access. It can be used
 for debugging and security analysis purposes.
 Please be aware that this will generate a lot of output.
 
-The plugin takes no argument::
+The plugin needs default argument::
 
   qemu-system-arm $(QEMU_ARGS) \
     -plugin ./contrib/plugins/libexeclog.so -d plugin
@@ -364,6 +364,13 @@ which will output an execution trace following this structure::
   0, 0xd34, 0xf9c8f000, "bl #0x10c8"
   0, 0x10c8, 0xfff96c43, "ldr r3, [r0, #0x44]", load, 0x200000e4, RAM
 
+the output can be filtered to only track certain instructions or
+addresses using the `ifilter` or `afilter` options. You can stack the
+arguments if required::
+
+  qemu-system-arm $(QEMU_ARGS) \
+    -plugin ./contrib/plugins/libexeclog.so,ifilter=st1w,afilter=0x40001808 -d plugin
+
 - contrib/plugins/cache.c
 
 Cache modelling plugin that measures the performance of a given L1 cache