[IA64] Fix initial value of cr.pta

The initial value of cr.pta in a vcpu is zero. So a Reserved
Register/Field fault is raised if a guest executes the following
sequence:

    mov r2=cr.pta;;
    ...
    mov cr.pta=r2

Actually, the windows installer with vcpus>=3 crashes due to this
issue.

Signed-off-by: Kouya Shimura <kouya@jp.fujitsu.com>

xen/arch/ia64/xen/vcpu.c

@@ -175,6 +175,9 @@ void vcpu_init_regs(struct vcpu *v)
 		VCPU(v, itv) = (1 << 16);	/* timer vector masked */
 	}
 
+	/* pta.size must not be 0.  The minimum is 15 (32k) */
+	VCPU(v, pta) = 15 << 2;
+
 	v->arch.domain_itm_last = -1L;
 }