qapi: add const prefix to 'char *' insider c_type()

qapi: add const prefix to 'char *' insider c_type()

It's ugly to add const prefix for parameter type by an if statement
outside c_type(). This patch adds a parameter to do it.

Signed-off-by: Amos Kong <akong@redhat.com>
Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>