storvsc: use shost_for_each_device() instead of open coding

Comment in struct Scsi_Host says that drivers are not supposed to access
__devices directly. storvsc_host_scan() doesn't happen in irq context
so we can just use shost_for_each_device().

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Long Li <longli@microsoft.com>
Acked-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>