blktap2: configurable driver chains

blktap2: configurable driver chains

Blktap2 allows block device drivers to be layered to create more
advanced virtual block devices. However, composing a layered driver is
not exposed to the user. This patch fixes this, and allows the user to
explicitly specify a driver chain when starting a tapdisk process,
using the pipe character ('|') to explicitly seperate layers in a
blktap2 configuration string.

for example, the command:
  ~$ tapdisk2 -n "log:|aio:/path/to/file.img"
will create a blktap2 device where read and write requests are passed
to the 'log' driver, then forwarded to the 'aio' driver.

Signed-off-by: Ryan O'Connor <rjo@cs.ubc.ca>