qemu-xen-traditional: disable docs

qemu-xen-traditional: disable docs

Perl 5.18 is unhappy with the qemu-xen-traditional documents, giving
this error:
  qemu.pod around line 91: Non-ASCII character seen before =encoding in
  'Schuetz.'. Assuming UTF-8 POD document had syntax errors at /usr/bin/core_pe\
rl/pod2man line 71.
  make[3]: *** [qemu.1] Error 255

We do not want these docs.  They are not really relevant to Xen users.
So instead of backporting the utf-8 fix from qemu upstream
(3179d694a8dcaa091131e3db644d445c0130713e), we just disable the docs
build.

We do this in xen-hooks.mak because qemu-xen-traditional's configure
script lacks the ability to explicitly disable the docs build.

(The docs build for upstream-based qemu-xen was already disabled by
xen.git#a0d110801d701c43e7b8c73dbd6b2444a10a7cdb)

Reported-by: jacek burghardt <jaceksburghardt@gmail.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com> (re 4.3 release)
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>