virt.ks: Add qemu-guest-agent package in RHEL6's kickstart file

qemu-guest-agent package exists in the cd image after RHEL6.3,
thus install it directly in guest installation stage.

For those RHEL6 guest before RHEL6.3, add a "ignoremissing"
option to let them ignore this new package.
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>
Acked-by: NFeng Yang <fyang@redhat.com>