qemu: explicitly disable xen support

qemu: explicitly disable xen support

We don't make use of xen and when building on Ubuntu 13.04 when
libxen-dev is installed on the build host you will get errors like the
following:

| /usr/include/x86_64-linux-gnu/bits/string3.h:81: warning: memset used with constant zero length parameter; this could be due to transposed parameters
| /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../lib/libxenguest.so: undefined reference to `lzma_alone_decoder@XZ_5.0'
| /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../lib/libxenguest.so: undefined reference to `lzma_code@XZ_5.0'
| /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../lib/libxenguest.so: undefined reference to `lzma_stream_decoder@XZ_5.0'
| /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../lib/libxenguest.so: undefined reference to `lzma_end@XZ_5.0'

This change disables xen for both -native and target packages but
since it is a PACKAGECONFIG a user could tune this to have xen support
in the target package.

(From OE-Core rev: fd638b975aac826d7137fd11db94b64ba82de592)

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Ming Liu <ming.liu@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>