[PATCH v2 1/3] Add cache_init() call to initial_xen().

Commit 92563d7a (cache: Allocate buffers
at initialization to detect malloc() failure) split cache_alloc() to
cache_init() and cache_alloc(). cache_init() is called in initial().
However, sadly initial_xen() is called before cache_init() and it uses
cache_alloc() indirectly which refers to uninitialized structures.
Hence, makedumfile run on Xen crash dumps finally fails. This patch adds
cache_init() call to initial_xen() and fixes above mentioned issue.
cache_init() is called in initial() if crash dump is not Xen one.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>