libxl: allow the storage backend to choose between qdisk/blkback

We remove the 'use_qdisk' global option because we want the storage
manager to decide what the backend should look like. Note that with
libxl we have 2 choices:

- blkback
- qdisk

There is no tapdisk3 support in libxl.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
libxl: store the VDI.attach result in xenstore

We need to know this value so we can predict the xenstore paths
used by the device. We need these paths so we can wait for hotplug.
We need to wait for hotplug because we can't distinguish between
a device being plugged in and one which is spotaneously unplugging.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
"make clean" should not delete the config.mk

It's very annoying to have to re-run configure after a "make clean".

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
make install: generate a xenopsd.conf that works

The script to generate the config file had got out of sync with
the code.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Fix "make install" and "make uninstall"

The "make install" now does everything that the manual install in
xen-api-libs-specs/xenopsd.spec.in does, except copying in the
2nd copy of network.conf (the one in %{_libexec}/xenopsd was being
ignored in favour of the one in /etc/xapi)

This should allow the .spec file to simply run "make install"

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #145 from djs55/libxl-qdisk

libxl: allow the storage backend to choose between qdisk/blkback