xen_disk: be consistent with use of xendev and blkdev->xendev

xen_disk: be consistent with use of xendev and blkdev->xendev

Certain functions in xen_disk are called with a pointer to xendev
(struct XenDevice *). They then use container_of() to acces the surrounding
blkdev (struct XenBlkDev) but then in various places use &blkdev->xendev
when use of the original xendev pointer is shorter to express and clearly
equivalent.

This patch is a purely cosmetic patch which makes sure there is a xendev
pointer on stack for any function where the pointer is need on multiple
occasions modified those functions to use it consistently.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>