dom0+vm: execute qrexec service as shell script

This will allow to pass quoted arguments and some variable expansion. Basically
one-line shell command service can be embeded in the configuration.
In previus version use of "command path with spaces" whould result in try to
execute '"command' with arguments 'path', 'with' and 'spaces"'.

qrexec/qubes_rpc_multiplexer

@@ -7,7 +7,7 @@ fi
 CFG_FILE=$QUBES_RPC/"$1"
 export QREXEC_REMOTE_DOMAIN="$2"
 if [ -s "$CFG_FILE" ] ; then
-	exec $(cat "$CFG_FILE")
+	exec /bin/sh "$CFG_FILE"
 	echo "$0: failed to execute handler for" "$1" >&2
 	exit 1
 fi