[BLKTAP] have blktap use a dynamic major

[BLKTAP] have blktap use a dynamic major
blktap currently uses a hardcoded major of 254 for the device.  This is
not robust in anyway and needs to be dynamic.

Note: it is better not to have the daemon create the node, and have udev
create it instead. But since the daemon currently creates the node
anyway, it is still the way this is done.  That change needs to be made
at another time.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>