[CORE] vmm_manager: Convert manager spin lock to mutex

Currently, we protect VCPU allocation, Guest allocation, and
VCPU iteration using global manager spin lock. This means
thread holding this global spin lock cannot be scheduled-out
or preempted.

This patch converts global manager spin lock into a mutex
lock so that threads holding it can be preempted. This will
make life lot simpler for writing load balancer where we
would require to iterate over all VCPUs.

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