tools/libxl: Fix build of libxl with ./configure --disable-blktap2

tools/libxl: Fix build of libxl with ./configure --disable-blktap2

Autoconf already has appropriate Linux/non-Linux logic for CONFIG_BLKTAP2.

Let LIBXL_BLKTAP inherit directly from CONFIG_BLKTAP2, rather than requiring a
further LIBXL_BLKTAP=n in addition to ./configure --disable-blktap2 to get
libxl to successfully link under Linux.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>