Add new operation XENOPROF_get_buffer in xenoprof hypercall. Also

Add new operation XENOPROF_get_buffer in xenoprof hypercall. Also
remove buffer related arguments from XENOPROF_init operation. This
is the first step to allow dynamic mapping/unmapping of xenoprof
buffers to
enable profiling a domain in passive mode and then switch to active
mode (or vice-versa). Currently a domain can only be profiled in a
single mode. Also passive domains cannot have oprofile builtin the
kernel or passive profiling will fail.

Signed-off-by: Jose Renato Santos <jsantos@hpl.hp.com>