net/virtio-user: support memory hotplug

net/virtio-user: support memory hotplug

When memory is hot-added or hot-removed, the virtio-user driver has to
notify the vhost-user backend with sending a VHOST_USER_SET_MEM_TABLE
request with the new memory map as payload.

This patch implements and registers a mem_event callback, it pauses the
datapath and updates memory regions to vhost in case of hot-add or
hot-remove event. This memory region update has only to be done when the
device is already started, so a new status flag is added to the device to
keep track of the status.

As the device can now be managed by different threads, a mutex is
introduced to protect against concurrent device configuration.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>