virt.test: skip replace floppy_ks in kernel_params

Reading ks from floppy driectly isn't works for some distr but boot option
'ks=hd:/dev/fd0' can reading ks from mounted floppy; so skip replace
this options;

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #849 from xutian/fix_floppy_ks

Small fix for unattended_install.floppy_ks test