dracut: fix qubes-vm module dependencies

It needs 'dm' for 'dmsetup' utility. Additionally fix check - qubesdb
path starts with '/'.

QubesOS/qubes-issues#1354

dracut/full-dmroot/module-setup.sh

@@ -1,13 +1,18 @@
 #!/bin/bash
 
 check() {
-    if xenstore-read qubes-vm-type &>/dev/null || qubesdb-read qubes-vm-type &>/dev/null; then
+    if xenstore-read qubes-vm-type &>/dev/null || qubesdb-read /qubes-vm-type &>/dev/null; then
         return 0
     else
         return 255
     fi
 }
 
+depends() {
+    echo dm
+    return 0
+}
+
 install() {
     inst_hook pre-udev 90 $moddir/qubes_cow_setup.sh
     inst_multiple \