arch_init: Simplify code for load_xbzrle()

For xbzrle_decode_buffer(), when decoding contents will exceed writing
buffer, it will return -1, so need not check the return value whether
large than writing buffer.

And when failure occurs within load_xbzrle(), it always return -1
without any resources which need release.

So can remove the related checking statements, and also can remove 'rc'
and 'ret' local variables,

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
