qemu.py: Use custom exceptions rather than Exception

The naked Exception should not be widely used. It makes sense to be a
bit more specific and use better-suited custom exceptions. As a benefit
we can store the full reply in the exception in case someone needs it
when catching the exception.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170818142613.32394-6-ldoktor@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>