qapi: add SysEmuTarget to "common.json"

We'll soon need an enumeration type that lists all the softmmu targets
that QEMU (the project) supports. Introduce @SysEmuTarget to
"common.json".

The enum constant @x86_64 doesn't match the QAPI convention of preferring
hyphen ("-") over underscore ("_"). This is intentional; the @SysEmuTarget
constants are supposed to produce QEMU executable names when stringified
and appended to the "qemu-system-" prefix. Put differently, the
replacement text of the TARGET_NAME preprocessor macro must be possible to
look up in the list of (stringified) enum constants.

Like other enum types, @SysEmuTarget too can be used for discriminator
fields in unions. For the @i386 constant, a C-language union member called
"i386" would be generated. On mingw build hosts, "i386" is a macro
however. Add "i386" to "polluted_words" at once.

Cc: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20180427192852.15013-3-lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/common.py

@@ -1822,7 +1822,7 @@ def c_name(name, protect=True):
                      'and', 'and_eq', 'bitand', 'bitor', 'compl', 'not',
                      'not_eq', 'or', 'or_eq', 'xor', 'xor_eq'])
     # namespace pollution:
-    polluted_words = set(['unix', 'errno', 'mips', 'sparc'])
+    polluted_words = set(['unix', 'errno', 'mips', 'sparc', 'i386'])
     name = name.translate(c_name_trans)
     if protect and (name in c89_words | c99_words | c11_words | gcc_words
                     | cpp_words | polluted_words):