rules: Temporarily disable nbdkit support

The situation with nbdkit/libnbd availability is a bit
complicated: it's not available on all architectures that
libvirt currently builds on, but the maintainers haven't
explicitly excluded any of those architectures, so in at least
some cases the setback might be just a temporary one.

Since nbdkit usage in libvirt is not currently permitted by the
AppArmor policy, we want to keep it disabled regardless.

debian/rules

@@ -23,6 +23,9 @@ ARCHES_LXC  = alpha amd64 arm64 armel armhf hppa i386 m68k mips64el mipsel power
 ARCHES_XEN  = amd64 arm64 armhf
 ARCHES_VBOX = amd64 i386
 
+# Keep this list empty until we have AppArmor support
+ARCHES_NBDKIT =
+
 ifeq ($(DEB_HOST_ARCH_OS), linux)
     WITH_STORAGE_LVM          = -Dstorage_lvm=enabled
     WITH_STORAGE_ISCSI        = -Dstorage_iscsi=enabled
@@ -99,6 +102,11 @@ ifneq (,$(filter $(DEB_HOST_ARCH), $(ARCHES_VBOX)))
 else
     WITH_VBOX                 = -Ddriver_vbox=disabled
 endif
+ifneq (,$(filter $(DEB_HOST_ARCH), $(ARCHES_NBDKIT)))
+    WITH_NBDKIT               = -Dnbdkit=enabled
+else
+    WITH_NBDKIT               = -Dnbdkit=disabled
+endif
 
 DEB_CONFIGURE_EXTRA_ARGS := \
     --libexecdir=/usr/lib/libvirt \
@@ -154,6 +162,7 @@ DEB_CONFIGURE_EXTRA_ARGS := \
     $(WITH_FIREWALLD) \
     $(WITH_ATTR) \
     $(WITH_NSS_PLUGIN) \
+    $(WITH_NBDKIT) \
     -Dwireshark_dissector=enabled \
     $(NULL)