meta-armv8r64-extras/demo: Register IRQ after vring initialized

Registering evtchn to metal irq before vring and vq are initialized
may trigger null pointer access in irq handler, and cause Segmentation
fault.

To avoid this potential risk, register evtchn to metal irq after vring
and vq are initialized.

Issue-Id: SCM-5451
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: I3b23cd6bd933098c159fe8b4d522c45944caf8fc