netdev-dpdk: vHost client mode and reconnect

netdev-dpdk: vHost client mode and reconnect

Until now, vHost ports in OVS have only been able to operate in 'server'
mode whereby OVS creates and manages the vHost socket and essentially
acts as the vHost 'server'. With this commit a new mode, 'client' mode,
is available. In this mode, OVS acts as the vHost 'client' and connects
to the socket created and managed by QEMU which now acts as the vHost
'server'. This mode allows for reconnect capability, which allows a
vHost port to resume normal connectivity in event of switch reset.

By default dpdkvhostuser ports still operate in 'server' mode. That is
unless a valid 'vhost-server-path' is specified for a device like so:

ovs-vsctl set Interface dpdkvhostuser0
options:vhost-server-path=/path/to/socket

'vhost-server-path' represents the full path of the vhost user socket
that has been or will be created by QEMU. Once specified, the port stays
in 'client' mode for the remainder of its lifetime.

QEMU v2.7.0+ is required when using OVS in vHost client mode and QEMU in
vHost server mode.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>