datapath: Change vport type from string to integer enumeration.

datapath: Change vport type from string to integer enumeration.

I plan to make the vport type part of the standard header stuck on each
Netlink message related to a vport.  As such, it is more convenient to use
an integer than a string.  In addition, by being fundamentally different
from strings, using an integer may reduce the confusion we've had in the
past over the differences in userspace and kernel names for network device
and vport types.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>