[CORE] vmm_workqueue: Allow vmm_work to be rescheduled when it is in-progress

There is no reason why we should not allow vmm_work to be rescheduled when
it is in-progress. This patch make re-scheduling of vmm_work more flexible.

Also, the system workqueue should not have higher priority then general
threads and Guest VCPUs. In future, many parts of Xvisor will use system
workqueue for deferring work hence we don't want such deferred work to
prempt threads and Guest VCPUs.

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