This patch makes blktap Do The Right Thing(TM).  It allows udev to 
create the /dev/xen/blktap[0-9] devices.

It creates a sysfs class called "xen".  This part may later be placed 
someplace else, but currently blktap is the only user so it is placed in 
the blktap code.

When blktap is initialized, a blktap0 sysfs class device is made.  The 
other devices blktapX (X > 0) are made when the BLKTAP_IOCTL_NEWINTF 
ioctl is called.  This way we don't flood the sysfs and /dev/xen with 
unnecessary devices.

I added a rule in the xen-backend.rules to allow for udev to create the 
blktap devices.


With this, we can really remove the code to search and create the 
/dev/xen/blktap[0-9]*, but I'll leave it in for now. With the use of 
udev, we really should remove that code as well as the code for creating 
the evtchn device.  udev works for both of these now.  But that removal 
will have to be in another patch.

-- Steve