Multiple virtio9p shares

[huguesalary]

This PR allows the user to specify multiple folders to share via virtio9p.

Previously, only /Users was being shared inside the guest at /Users.

With this PR, it is possible to mount any Host folder inside the Guest at /xhyve-virtio9p (by default). This default can be changed via the --xhyve-virtio-9p-root command line argument.

This commit changes --xhyve-virtio-9p from a bool flag to a string slice representing the absolute path to the Host folder to be mounted inside the guest.

This PR is divided in 2 commits:

• The first one is the actual code allowing multiple Virtio9p
• The second commit fixes a bug preventing a user to use both NFS and Virtio9p

Here's an example of how to use:

docker-machine -D create -d xhyve --xhyve-disk-size 10000 --xhyve-memory-size 4000 --xhyve-cpu-count 2 --xhyve-virtio-9p /Users --xhyve-virtio-9p /usr --xhyve-virtio-9p-root /myVirtio9pRoot test

This command will share the host folders /Users and /usr in the guest at /myVirtio9pRoot/Users and /myVirtio9pRoot/usr