kata_agent: use virtio-fs 0.3+ mount options

virtio-fs changed the mount command-line.  Previously "mount none -o
tag=kataShared ..." was used.  Now "mount kataShared ..." is used
instead.

Since the "kataShared" tag is used for both 9P and virtio-fs, rename the
variable so that it is not 9P-specific.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>