API: make declaration of _LAST enum values conditional

Although this is a public API break, it only affects users that
were compiling against *_LAST values, and can be trivially
worked around without impacting compilation against older
headers, by the user defining VIR_ENUM_SENTINELS before using
libvirt.h.  It is not an ABI break, since enum values do not
appear as .so entry points.  Meanwhile, it prevents users from
using non-stable enum values without explicitly acknowledging
the risk of doing so.

See this list discussion:
https://www.redhat.com/archives/libvir-list/2012-January/msg00804.html

* include/libvirt/libvirt.h.in: Hide all sentinels behind
LIBVIRT_ENUM_SENTINELS, and add missing sentinels.
* src/internal.h (VIR_DEPRECATED): Allow inclusion after
libvirt.h.
(LIBVIRT_ENUM_SENTINELS): Expose sentinels internally.
* daemon/libvirtd.h: Use the sentinels.
* src/remote/remote_protocol.x (includes): Don't expose sentinels.
* python/generator.py (enum): Likewise.
* tests/cputest.c (cpuTestCompResStr): Silence compiler warning.
* tools/virsh.c (vshDomainStateReasonToString)
(vshDomainControlStateToString): Likewise.

python/generator.py

@@ -205,7 +205,8 @@ def enum(type, name, value):
         value = 1
     elif value == 'VIR_DOMAIN_AFFECT_CONFIG':
         value = 2
-    enums[type][name] = value
+    if name[-5:] != '_LAST':
+        enums[type][name] = value
 
 def qemu_enum(type, name, value):
     if not qemu_enums.has_key(type):