xentrace: Per-cpu xentrace buffers

xentrace: Per-cpu xentrace buffers

In the current xentrace configuration, xentrace buffers are all
allocated in a single contiguous chunk, and then divided among logical
cpus, one buffer per cpu.  The size of an allocatable chunk is fairly
limited, in my experience about 128 pages (512KiB).  As the number of
logical cores increase, this means a much smaller maximum per-cpu
trace buffer per cpu; on my dual-socket quad-core nehalem box with
hyperthreading (16 logical cpus), that comes to 8 pages per logical
cpu.

This patch addresses this issue by allocating per-cpu buffers
separately.

Signed-off-by: George Dunlap <dunlapg@umich.edu>