xen/arm: support the ARM generic virtual timer

xen/arm: support the ARM generic virtual timer

Save and restore the virtual timer registers during the context switch.
At save time initialize an internal Xen timer to make sure that Xen
schedules the guest vcpu at the time of the next virtual timer
interrupt.
Receive the virtual timer interrupt into the hypervisor and inject it
into the running guest.

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