qemu/queue.h: typedef QTAILQ heads

qemu/queue.h: typedef QTAILQ heads

This will be needed when we change the QTAILQ head and elem structs
to unions.  However, it is also consistent with the usage elsewhere
in QEMU for other list head structs (see for example FsMountList).

Note that most QTAILQs only need their name in order to do backwards
walks.  Those do not break with the struct->union change, and anyway
the change will also remove the need to name heads when doing backwards
walks, so those are not touched here.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>