xen/vtimer: fixes and improvements

xen/vtimer: fixes and improvements

Do not try to save and restore the vtimer for the idle domain.

Inject the vtimer interrupt from the Xen timer handler, taking care of
setting the timer as masked in the ctl field, so that at restore time it
is not going to fire the interrupt again.

No need to disable the vtimer before writing the new offset on restore:
the vtimer is already disabled.

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>