qapi: force a UTF-8 locale for running Python

Python2 did not validate locale correctness when reading input data, so
would happily read UTF-8 data in non-UTF-8 locales. Python3 is strict so
if you try to read UTF-8 data in the C locale, it will raise an error
for any UTF-8 bytes that aren't representable in 7-bit ascii encoding.
e.g.

UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 54: ordinal not in range(128)
Traceback (most recent call last):
File "/tmp/qemu-test/src/scripts/qapi-commands.py", line 317, in <module>
schema = QAPISchema(input_file)
File "/tmp/qemu-test/src/scripts/qapi.py", line 1468, in __init__
parser = QAPISchemaParser(open(fname, 'r'))
File "/tmp/qemu-test/src/scripts/qapi.py", line 301, in __init__
previously_included)
File "/tmp/qemu-test/src/scripts/qapi.py", line 348, in _include
exprs_include = QAPISchemaParser(fobj, previously_included, info)
File "/tmp/qemu-test/src/scripts/qapi.py", line 271, in __init__
self.src = fp.read()
File "/usr/lib64/python3.5/encodings/ascii.py", line 26, in decode
return codecs.ascii_decode(input, self.errors)[0]

More background on this can be seen in

https://www.python.org/dev/peps/pep-0538/

Many distros support a new C.UTF-8 locale that is like the C locale,
but with UTF-8 instead of 7-bit ASCII. That is not entirely portable
though. This patch thus sets the LANG to "C", but overrides LC_CTYPE
to be en_US.UTF-8 locale. This gets us pretty close to C.UTF-8, but
in a way that should be portable to everywhere QEMU builds.

This patch only forces UTF-8 for QAPI scripts, since that is the one
showing the immediate error under Python3 with C locale, but potentially
we ought to force this for all python scripts used in the build process.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20180116134217.8725-9-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>