datapath-windows: Add support for OVS_DP_CMD_NEW Netlink command.

In this change, we add support for the 'OVS_DP_CMD_NEW' netlink command
in the kernel. We don't really support creation of a new datapath. If
the name of the userspace is the same as the single datapath we support,
we return EEXIST.

This code is required to bootstrap ovs-vswitchd which makes the
following sequence of calls:
open_dpif_backer() -> dpif_create_and_open() -> dpif_create()

We also rename HandleDpTransaction() to HandleDpTransactionCommon().

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Tested-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>