net/virtio-user: fix deadlock in memory events callback

net/virtio-user: fix deadlock in memory events callback

Deadlock can occur when allocating memory if a vhost-kernel
based virtio-user device is in use. To fix the deadlock,
we will take memory hotplug lock explicitly in virtio-user
when necessary, and always call the _thread_unsafe memory
functions.

Bugzilla ID: 81
Fixes: 12ecb2f63b12 ("net/virtio-user: support memory hotplug")
Cc: stable@dpdk.org
Reported-by: Seán Harte <seanbh@gmail.com>
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Tested-by: Seán Harte <seanbh@gmail.com>
Reviewed-by: Seán Harte <seanbh@gmail.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>