tools: remove blktap2 related code and documentation

Blktap2 is effectively dead code for a few years.

Notable changes in this patch:

0. Unhook blktap2 from build system
1. Now libxl no longer supports TAP disk backend, appropriate assertions
   are added and some code paths now return ERROR_FAIL
2. Tap is no longer a supported backend in doc
3. Remove relevant entries in MAINTAINERS

A patch to actually remove blktap2 directory will come later.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <George.Dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/config.h.in

@@ -3,9 +3,6 @@
 /* Enabling support partial device tree in libxl */
 #undef ENABLE_PARTIAL_DEVICE_TREE
 
-/* Blktap2 enabled */
-#undef HAVE_BLKTAP2
-
 /* Define to 1 if you have the declaration of `fdt_first_subnode', and to 0 if
    you don't. */
 #undef HAVE_DECL_FDT_FIRST_SUBNODE
@@ -27,9 +24,6 @@
 /* Define to 1 if you have the <inttypes.h> header file. */
 #undef HAVE_INTTYPES_H
 
-/* Define to 1 if you have the `aio' library (-laio). */
-#undef HAVE_LIBAIO
-
 /* Define to 1 if you have the `crypto' library (-lcrypto). */
 #undef HAVE_LIBCRYPTO