dracut: change the way to include ehci-pci module

Apparently add_drivers doesn't work. Looking at kernel-modules dracut
code, it can only be used for block-device driver and only makes sense
in --host-only mode.
So add additional module, which unconditionally install kernel modules.

dracut/dracut.conf.d/extra-modules.conf

@@ -0,0 +1 @@
+add_dracutmodules+=" extra-modules "

dracut/dracut.conf.d/usb-drivers.conf

@@ -1 +0,0 @@
-add_drivers+=" ehci-pci "

dracut/modules.d/90extra-modules/module-setup.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+# Install some missing modules
+
+installkernel() {
+    # ehci-hcd split off
+    instmods ehci-pci ehci-platform || :
+}

rpm_spec/core-dom0-linux.spec

@@ -217,6 +217,8 @@ mv -f /lib/udev/rules.d/69-xorg-vmmouse.rules /var/lib/qubes/removed-udev-script
 /etc/dracut.conf.d/*
 %dir %{_dracutmoddir}/90qubes-pciback
 %{_dracutmoddir}/90qubes-pciback/*
+%dir %{_dracutmoddir}/90extra-modules
+%{_dracutmoddir}/90extra-modules/*
 # Qrexec
 %attr(4750,root,qubes) /usr/lib/qubes/qrexec-daemon
 /usr/lib/qubes/qrexec-client