[xs64bit] CA-141723: Allocate one evtchn per-VCPU in alignment hack

When allocating event channels at start-of-day for HVM guests so that they're
backwards-compatible with old PV drivers (see c1a4c42), make sure we allocate
one event channel for each VCPU since each VCPU will have an IO req. event
channel bound.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>