vmm, pci: Implement virtio-mem support for vfio-user

Implement the infrastructure that lets a virtio-mem device map the guest
memory into the device. This is necessary since with virtio-mem zones
memory can be added or removed and the vfio-user device must be
informed.

Fixes: #3025

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

virtio-devices/src/seccomp_filters.rs

@@ -123,6 +123,8 @@ fn virtio_mem_thread_rules() -> Vec<(i64, Vec<SeccompRule>)> {
     vec![
         (libc::SYS_fallocate, vec![]),
         (libc::SYS_ioctl, create_virtio_mem_ioctl_seccomp_rule()),
+        (libc::SYS_recvfrom, vec![]),
+        (libc::SYS_sendmsg, vec![]),
     ]
 }