VM feature: improve sed command line to replace root=... kernel option

Quoting bugreport:

| [...] I found an issue which renders
| every preconfigured BOOTPARAMS for vmfiles useless.
|
| Near the end of grml-debootstrap the grub config is changed to boot in
| virtual environments. Unfortunately the whole line is replaced to
| root=/dev/sda1 and each parameter that was previously set is gone by
| then.
|
| See attached a diff that just replaces the root device string.

Thanks: Conny Seidel <conny.seidel@amd.com> for bugreport + patch