vhost_user: cleanup mem_table when

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
vhost_user: implement stop()

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Merge pull request #291 from virtualopensystems/vhost_user_stop

Vhost user stop
receive checksum offload
transmit checksum offload
the "insert checksum" bits of TXDESC must reflect the current context
precalculate the offload flags together with the context descriptor
check if the packet needs checksum before asking the chip to do it
add offload support for SCTP
test: start sending doctored packets to see if bad checksums are detected and if good ones can be inserted