ioemu: Add tapdisk-ioemu tool

ioemu: Add tapdisk-ioemu tool

Currently, tap:ioemu can only be used for domains which have a device
model running. This isn't the case for all domains. The most important
of the missing domains is Dom0 which needs acces e.g. to extract the
kernel from the domain's image.

tapdisk-ioemu is a tool compiled from ioemu source plus a small
wrapper which handles tap:ioemu access for domains without device
model (currently Dom0). You must start tapdisk-ioemu manually before
trying to attach a tap:ioemu disk to Dom0 at the moment. A patch to
blktapctrl will follow to automatically start tapdisk-ioemu when
needed.

Signed-off-by: Kevin Wolf <kwolf@suse.de>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>