Rename struct device_context and re-arrange the fields inside.

Rename struct device_context to struct vm_device, and move struct device
field to the end according to Document/driver-model standard.
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>