Use ioemu block drivers through blktap; import other Xen-specific files.

Use ioemu block drivers through blktap; import other Xen-specific files.

Add support for a tap:ioemu pseudo driver. Devices using this driver
won't use tapdisk (containing the code duplication) any more, but will
connect to the qemu-dm of the domain. In this way no working
configuration should be broken right now as you can still choose to
use the tapdisk drivers.

Signed-off-by: Kevin Wolf <kwolf@suse.de>
Also, import and update various Xen-specific files from xen-unstable
tip 17307:b667e220e556.