Use blktap only for VHD files

When libxl chooses which backend type to use for a disk which is
LIBXL_DISK_BACKEND_UNKNOWN, use blktap only for VHD format disks
and defer to qemu for all other formats, including RAW.

There are two advantages:
  1. blktap/tapdisk is particularly good at handling VHD files, so
     we still use that
  2. qemu has high-quality support for a larger set of file formats
     (e.g. QCOW2) and network-based disk access protocols (such as
     ceph/RBD, user-space iSCSI etc) which we also get to use

Signed-off-by: David Scott <dave.scott@eu.citrix.com>