kvmtool/tests: fix iso build on debian

Debian and some other distro's don't provide mkisofs due to
licensing concerns. xorrisofs from package xorriso provides
a command-line compatible command in this case. Update the
makefile of tests to pick xorrisofs if mkisofs is not available.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>