libxl: sane disk backend selection and validation

libxl: sane disk backend selection and validation

Introduce a new function libxl__device_disk_set_backend which
does some sanity checks and determines which backend ought to be used.

If the caller specifies LIBXL_DISK_BACKEND_UNKNOWN (which has the
value 0), it tries PHY, TAP and QDISK in that order.  Otherwise it
tries only the specified value.

libxl__device_disk_set_backend (and its helper function
disk_try_backend) inherit the role (and small amounts of the code)
from validate_virtual_disk.  This is called during do_domain_create
and also from libxl_disk_device_add (for the benefit of hotplug
devices).

It also now takes over the role of the scattered fragments of backend
selection found in libxl_device_disk_add,
libxl_device_disk_local_attach and libxl__need_xenpv_qemu.  These
latter functions now simply do the job for the backend they find has
already been specified and checked.

The restrictions on the capabilities of each backend, as expressed in
disk_try_backend (and to an extent in libxl_device_disk_local_attach)
are intended to be identical to the previous arrangements.

In 23618:3173b68c8a94 combined with 23622:160f7f39841b,
23623:c7180c353eb2, "xl" effectively became much more likely to select
TAP as the backend.  With this change to libxl the default backend
selected by the libxl__device_disk_set_backend is intended to once
again to be PHY where possible.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>