Fix wrong variable name

Commit bc5d0215 added support for disk params, but due to copy-paste
it tries to enforce the hvparams into disk params values, leading to:

$ gnt-cluster modify -H kvm:initrd_path=/boot/initrd-2.6-kvmU
Parameter Error: Unknown parameter 'initrd_path'

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Andrea Spadaccini <spadaccio@google.com>