xen: change the type of xen_vcpu_id to uint32_t

xen: change the type of xen_vcpu_id to uint32_t

We pass xen_vcpu_id mapping information to hypercalls which require
uint32_t type so it would be cleaner to have it as uint32_t. The
initializer to -1 can be dropped as we always do the mapping before using
it and we never check the 'not set' value anyway.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit 55467dea2967259f21f4f854fc99d39cc5fea60e)
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Orabug: 24820937