vswitchd: Introduce 'mtu_request' column in Interface.

vswitchd: Introduce 'mtu_request' column in Interface.

The 'mtu_request' column can be used to set the MTU of a specific
interface.

This column is useful because it will allow changing the MTU of DPDK
devices (implemented in a future commit), which are not accessible
outside the ovs-vswitchd process, but it can be used for kernel
interfaces as well.

The current implementation of set_mtu() in netdev-dpdk is removed
because it's broken.  It will be reintroduced by a subsequent commit on
this series.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>