netdev: Add 'errp' to set_config().

netdev: Add 'errp' to set_config().

Since 55e075e65ef9("netdev-dpdk: Arbitrary 'dpdk' port naming"),
set_config() is used to identify a DPDK device, so it's better to report
its detailed error message to the user.  Tunnel devices and patch ports
rely a lot on set_config() as well.

This commit adds a param to set_config() that can be used to return
an error message and makes use of that in netdev-dpdk and netdev-vport.

Before this patch:

$ ovs-vsctl add-port br0 dpdk0 -- set Interface dpdk0 type=dpdk
ovs-vsctl: Error detected while setting up 'dpdk0': dpdk0: could not set
    configuration (Invalid argument).  See ovs-vswitchd log for details.
ovs-vsctl: The default log directory is "/var/log/openvswitch/".

$ ovs-vsctl add-port br0 p+ -- set Interface p+ type=patch
ovs-vsctl: Error detected while setting up 'p+': p+: could not set
    configuration (Invalid argument).  See ovs-vswitchd log for details.
ovs-vsctl: The default log directory is "/var/log/openvswitch/".

$ ovs-vsctl add-port br0 gnv0 -- set Interface gnv0 type=geneve
ovs-vsctl: Error detected while setting up 'gnv0': gnv0: could not set
    configuration (Invalid argument).  See ovs-vswitchd log for details.
ovs-vsctl: The default log directory is "/var/log/openvswitch/".

After this patch:

$ ovs-vsctl add-port br0 dpdk0 -- set Interface dpdk0 type=dpdk
ovs-vsctl: Error detected while setting up 'dpdk0': 'dpdk0' is missing
    'options:dpdk-devargs'. The old 'dpdk<port_id>' names are not
    supported.  See ovs-vswitchd log for details.
ovs-vsctl: The default log directory is "/var/log/openvswitch/".

$ ovs-vsctl add-port br0 p+ -- set Interface p+ type=patch
ovs-vsctl: Error detected while setting up 'p+': p+: patch type requires
    valid 'peer' argument.  See ovs-vswitchd log for details.
ovs-vsctl: The default log directory is "/var/log/openvswitch/".

$ ovs-vsctl add-port br0 gnv0 -- set Interface gnv0 type=geneve
ovs-vsctl: Error detected while setting up 'gnv0': gnv0: geneve type
    requires valid 'remote_ip' argument.  See ovs-vswitchd log for
    details.
ovs-vsctl: The default log directory is "/var/log/openvswitch/".

CC: Ciara Loftus <ciara.loftus@intel.com>
CC: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Tested-by: Ciara Loftus <ciara.loftus@intel.com>