blktap2: Separate tapdisk raw I/O into different backends.

blktap2: Separate tapdisk raw I/O into different backends.

Hide tapdisk support for different raw I/O interfaces behind a new
struct tio. Libaio remains to dominate the interface, requiring
everyone to dispatch iocb/ioevent structs.

Backends:
 - lio:  Kernel AIO via libaio.
 - rwio: Canonical read/write() mode.

Misc:
 - Fixes a bug in tapdisk-vbd which locks up the sync io mode.
 - Wants a PERROR macro in blktaplib.h
 - Removes dead code in qcow2raw to make it link again.

Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
Signed-off-by: Jake Wires <jake.wires@citrix.com>