yocto/virtualization: Fix DomU1 vCPU pinning

Currently, the xl.cfg configuration for the DomU1 is wrongly
assigning the vCPU0 to run on either the pCPU 1 or 2, instead
what was intended was to pin both vCPU0 and vCPU1 to the pCPU1
and pCPU2 respectively.

Correct the xl.cfg syntax to have that behaviour.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>