qvm-copy-to-vm: fix handling empty target VM

This means "default" in Qubes 4.0.

QubesOS/qubes-issues#910

qubes-rpc/qvm-copy-to-vm

@@ -41,7 +41,7 @@ if [ $PROGRESS_TYPE = console ] ; then
 	export FILECOPY_TOTAL_SIZE=$(du --apparent-size -c -- "$@" 2> /dev/null | tail -1 | cut -f 1)
 fi
 
-/usr/lib/qubes/qrexec-client-vm $VM qubes.Filecopy /usr/lib/qubes/qfile-agent "$@"
+/usr/lib/qubes/qrexec-client-vm "$VM" qubes.Filecopy /usr/lib/qubes/qfile-agent "$@"
 
 if [ "${0##*/}" = "qvm-move-to-vm" ]; then
 	rm -rf -- "$@"