tools: fix compile errors with -Og

tools: fix compile errors with -Og

At least gcc-4.8 and older fails to recognize that err is always
initialized, the build fails:
  xc_cpupool.c: In function 'xc_cpupool_removecpu':
  xc_cpupool.c:168:5: error: 'err' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    return err;

Fix this in blktap2 and libxc.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>