xenbus: Use dev_printk() when possible

Use dev_printk() when possible to include device and driver information in
the conventional format.

Add "#define dev_fmt" to preserve KBUILD_MODNAME in messages.

No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20200527174326.254329-3-helgaas@kernel.org
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>