torture: Capture qemu output

Currently qemu output appears on standard output, but is inaccessible
later on.  This commit therefore captures this output and causes
kvm-recheck.sh to output this output if QEMU gave a non-zero non-137
exit code.  (And exit code of 137 indicates that QEMU was killed, in
which case we want to know about the hang rather than the fact that
QEMU was killed.)

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>