xen/arm: initialize virt_timer and phys_timer with the same values on all vcpus

xen/arm: initialize virt_timer and phys_timer with the same values on all vcpus

Introduce a domain wide vtimer initialization function to initialize
the phys_timer and the virt_timer offsets.

Use the domain phys_timer and virt_timer offsets throughout the vtimer
code instead of the per-vcpu offsets.

Remove the per-vcpu offsets from struct vtimer altogether.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>