xen-pciback: Add name prefix to global 'permissive' variable

The variable for the 'permissive' module parameter used to be static
but was recently changed to be extern.  This puts it in the kernel
global namespace if the driver is built-in, so its name should begin
with a prefix identifying the driver.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: af6fc858 ("xen-pciback: limit guest control of command register")
Signed-off-by: David Vrabel <david.vrabel@citrix.com>