libxenguest: simplify kernel decompression

libxenguest: simplify kernel decompression

In all cases the kernel build makes available the uncompressed size in
the final 4 bytes of the bzImage payload. Utilize this to avoid
repeated realloc()ing of the output buffer.

As a side effect this also addresses the previous mistaken return of 0
(success) from xc_try_{bzip2,lzma,xz}_decode() in case
xc_dom_register_external() would have failed.

As another side effect this also addresses the first error path of
_xc_try_lzma_decode() previously bypassing lzma_end().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wl@xen.org>