xen: add real time scheduler rt

This scheduler follows the pre-emptive Global EDF theory in real-time field.
Each VCPU can have a dedicated period and budget.
While scheduled, a VCPU burns its budget.
A VCPU has its budget replenished at the beginning of each of its periods;
The VCPU discards its unused budget at the end of each of its periods.
If a VCPU runs out of budget in a period, it has to wait until next period.
The mechanism of how to burn a VCPU's budget depends on the server mechanism
implemented for each VCPU.

Server mechanism: a VCPU is implemented as a deferable server.
When a VCPU is scheduled to execute on a PCPU, its budget is continuously
burned.

Priority scheme: Preemptive Global Earliest Deadline First (gEDF).
At any scheduling point, the VCPU with earliest deadline has highest
priority.

Queue scheme: A global Runqueue for each CPU pool.
The Runqueue holds all runnable VCPUs.
VCPUs in the Runqueue are divided into two parts: with and without budget.
At each part, VCPUs are sorted based on gEDF priority scheme.

Scheduling quantum: 1 ms;

Note: cpumask and cpupool is supported.

This is still in the development phase.

Signed-off-by: Sisu Xi <xisisu@gmail.com>
Signed-off-by: Meng Xu <mengxu@cis.upenn.edu>
libxc: add rt scheduler

Add xc_sched_rt_* functions to interact with Xen to set/get domain's
parameters for rt scheduler.

Signed-off-by: Sisu Xi <xisisu@gmail.com>
Signed-off-by: Meng Xu <mengxu@cis.upenn.edu>
libxl: add rt scheduler

Add libxl functions to set/get domain's parameters for rt scheduler

Signed-off-by: Sisu Xi <xisisu@gmail.com>
Signed-off-by: Meng Xu <mengxu@cis.upenn.edu>
xl: introduce rt scheduler

Add xl command for rt scheduler

Signed-off-by: Sisu Xi <xisisu@gmail.com>
Signed-off-by: Meng Xu <mengxu@cis.upenn.edu>