qapi: Fix C identifiers generated for names containing '.'

c_fun() maps '.' to '_', c_var() doesn't.  Nothing prevents '.' in
QAPI names that get passed to c_var().

Which QAPI names get passed to c_fun(), to c_var(), or to both is not
obvious.  Names of command parameters and struct type members get
passed to c_var().

c_var() strips a leading '*', but this cannot happen.  c_fun()
doesn't.

Fix c_var() to work exactly like c_fun().

Perhaps they should be replaced by a single mapping function.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
[add 'import string']
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
