torture: make kvm-find-errors.sh check for compressed vmlinux files

Under some conditions, a given run's vmlinux file will be compressed,
so that it is named vmlinux.xz rather than vmlinux.  in such cases,
kvm-find-errors.sh will complain about the nonexistence of vmlinux.
This commit therefore causes kvm-find-errors.sh to check for vmlinux.xz
as well as for vmlinux.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>