KVM: ia64: Remove lock held by halted vcpu

Remove the lock protection for kvm halt logic, otherwise,
once other vcpus want to acquire the lock, and they have to
wait all vcpus are waken up from halt.

Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>