KVM: ia64: Fix halt emulation logic

Common halt logic was changed by x86 and did not update ia64.  This patch
updates halt for ia64.

Fixes a regression causing guests to hang with more than 2 vcpus.

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