qapi: More idiomatic string operations

Rather than slicing the end of a string, we can use python's
endswith().  And rather than creating a set of characters,
we can search for a character within a string.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1445898903-12082-3-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>