[CORE] Introduce VCPU scheduler statistics for time accounting

This patch introduces state-level time statistics for each VCPU
to keep track of time spend by VCPU in various states such as
READY, RUNNING, PAUSED, and HALTED.

The state-level time statistics of a VCPU are resetted whenever
the VCPU is reset.

The most important use of state-level time statistics will be to
help implement meaningful load-balancing algorithms for SMP host.

We also update arch_vcpu_stat_dump() and arch_vcpu_regs_dump()
to provide character device used for printing as argument.

Signed-off-by: Anup Patel <anup@brainfault.org>