core: Add support for per-CPU statistics

Record VM exits on a per-CPU basis and export this information via the
"CPU Get Info" hypercall.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
driver: Export cell statistics via sysfs

Provide an accumulation of per-CPU statistics for each cell via sysfs.
This can help analyzing potential performance issues of Jailhouse cells.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>