I don't think there are any dom0's around whose kernels support only blktap1 and not something newer like blktap2 or qdisk. Certainly not that you would want to run Xen 4.4 on.
libxl will never use blktap1.
76c66c6 tools: disable blktap1 build by default
config/Tools.mk.in | 1 +
tools/Makefile | 4 ++--
tools/configure | 26 ++++++++++++++++++++++++++
tools/configure.ac | 1 +
tools/hotplug/Linux/Makefile | 4 +++-
5 files changed, 33 insertions(+), 3 deletions(-)