vm/initramfs: exclude plymouth module

It doesn't make any sense on hvc console, but causes problems.

Problem: plymouth-quit.service is masked in VM, so plymouthd remains
running controlling console.

kernel.spec

@@ -324,7 +324,7 @@ mkdir -p %buildroot/%vm_install_dir
 /sbin/dracut --nomdadmconf --nolvmconf \
     --kmoddir %buildroot/lib/modules/%kernelrelease \
     --include %_sourcedir/vm-initramfs / \
-    --add "dm" \
+    --add "dm" --omit "plymouth" \
     -d "xenblk xen-blkfront cdrom ext4 jbd2 crc16 dm_snapshot" \
     %buildroot/%vm_install_dir/initramfs %kernelrelease