lib/launch-direct.c: Simplify test for KVM, remove qemu caching

Previously we tested if KVM was available, and cached that for the
qemu binary.  I think this was actually wrong.  For example, if the
machine restarts, then the cache is still around, but KVM might be
enabled or disabled because of a new host kernel.

In any case, let's radically simplify this.

Test for KVM on each run.  Consequently we can remove all the qemu
test caching stuff as it is no longer used anywhere.

I also tightened up the code that runs the QMP query-kvm command, so
now any unexpected output will cause a runtime failure.  This command
ought to work, and if it breaks we ought to know about it and fix it.

tests/disks/debug-qemu.sh

@@ -23,6 +23,18 @@ if [ -z "$DEBUG_QEMU_FILE" ]; then
     exit 1
 fi
 
+# The direct backend runs qemu ... -qmp stdio to query for KVM.  For
+# the test to pass we have to provide an answer here.
+if [ "x$5" = "x-qmp" ]; then
+    # Consume stdin first.
+    cat >/dev/null
+    # Write some fake output.
+    echo '"QMP"'
+    echo '"return"'
+    echo '{"return": {"enabled": true, "present": true}}'
+    exit 0
+fi
+
 echo "$@" > "$DEBUG_QEMU_FILE"
 
 # Real qemu would connect back to the daemon socket with a working

tests/disks/test-qemu-drive.sh

@@ -33,6 +33,7 @@ function check_output ()
         echo "$0: guestfish command failed, see previous error messages"
         exit 1
     fi
+    cat "$DEBUG_QEMU_FILE"
 }
 
 function fail ()