vlog: Provide convenience macros to additionally store log messages in buffer

Signed-off-by: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
vswitchd: Add error column to Interface table to store error condition

Store the error condition of a failed port configuration in a new
column 'error' in the Interface table.

Example:
$ ovs-vsctl add-port br0 test -- \
     set Interface test type=vxlan options:unknown=1
ovs-vsctl: Error detected while setting up 'test'.  [...]

$ ovs-vsctl list Interface test | grep error
error         : "test: could not set configuration (Invalid argument)"

Fixing the error will clear the error column:
$ ovs-vsctl set Interface test options:remote_ip=1.1.1.1
$ ovs-vsctl list Interface test | grep error
error         : []
$

For now, the high level error messages when opening and configuring
the netdev are used. Further patches can extend passing the error
pointer into the individual netdev implementations to allow for more
fine grained error messages to be stored.

Signed-off-by: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
ovs-vsctl: Add error column to show command

a425a102-c317-4743-b0ba-79d59ff04a74
    Bridge "br0"
        [...]
        Port test
            Interface test
                type: vxlan
                options: {unknown="1"}
                error: "test: could not set configuration (Invalid argument)"
    ovs_version: "2.1.90"

Signed-off-by: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>