conf: Drop support for 'sheepdog' disks

conf: Drop support for 'sheepdog' disks

In upcoming patches we'll update minimum supported qemu version to
qemu-6.2 which no longer supports 'sheepdog'. This was the only
hypervisor driver that supported it.

Reject any config containing sheepdog disks when validating the XML,
remove the positive test cases in qemu and replace them by a negative
test case. This will still excercise the XML schema, but will prepare
for removal of the internal code.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>