xen: mask event channels while binding them to a vCPU

Mask the event channel source before trying to bind it to a CPU, this
prevents stray interrupts from firing while assigning them and hitting the
KASSERT in xen_intr_handle_upcall.

Sponsored by: Citrix Systems R&D


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275678 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f