qapi: Prefer single-quoted strings more consistently

qapi: Prefer single-quoted strings more consistently

PEP 8 advises:

    In Python, single-quoted strings and double-quoted strings are the
    same.  This PEP does not make a recommendation for this.  Pick a
    rule and stick to it.  When a string contains single or double
    quote characters, however, use the other one to avoid backslashes
    in the string.  It improves readability.

The QAPI generators succeed at picking a rule, but fail at sticking to
it.  Convert a bunch of double-quoted strings to single-quoted ones.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-20-git-send-email-armbru@redhat.com>