vhost-user: Support operation without MRG_RXBUF.
vhost-user: Support operation without MRG_RXBUF.

MRG_RXBUF is enabled by default in the beginning, and QEMU will initially
negotiate a feature set with Snabb NFV that includes MRG_RXBUF.  This
adds a field onto the virtio header, in legacy mode.  However if we
later negotiate to not have MRG_RXBUF, we need to re-set this value to
not have the extra fields.

(cherry picked from commit df8e83c6771a7b892324922f25d5123a69f651d9)

mixflowtech/logsensor,wingo/snabbswitch,eugeneia/snabbswitch,eugeneia/snabbswitch,snabbco/snabb,Igalia/snabbswitch,SnabbCo/snabbswitch,wingo/snabb,dpino/snabb,wingo/snabb,dpino/snabb,heryii/snabb,SnabbCo/snabbswitch,kbara/snabb,alexandergall/snabbswitch,mixflowtech/logsensor,eugeneia/snabbswitch,alexandergall/snabbswitch,Igalia/snabb,dpino/snabb,snabbco/snabb,alexandergall/snabbswitch,eugeneia/snabb,dpino/snabbswitch,eugeneia/snabbswitch,wingo/snabbswitch,Igalia/snabbswitch,heryii/snabb,wingo/snabb,alexandergall/snabbswitch,heryii/snabb,heryii/snabb,Igalia/snabb,SnabbCo/snabbswitch,snabbco/snabb,heryii/snabb,dpino/snabb,kbara/snabb,eugeneia/snabb,wingo/snabb,snabbco/snabb,Igalia/snabbswitch,mixflowtech/logsensor,snabbco/snabb,dpino/snabb,dpino/snabb,dpino/snabbswitch,eugeneia/snabb,snabbco/snabb,wingo/snabbswitch,snabbco/snabb,SnabbCo/snabbswitch,alexandergall/snabbswitch,wingo/snabb,Igalia/snabb,eugeneia/snabb,heryii/snabb,kbara/snabb,kbara/snabb,mixflowtech/logsensor,kbara/snabb,eugeneia/snabb,eugeneia/snabb,wingo/snabb,kbara/snabb,snabbco/snabb,alexandergall/snabbswitch,Igalia/snabb,alexandergall/snabbswitch,dpino/snabbswitch,Igalia/snabbswitch,Igalia/snabb,mixflowtech/logsensor,eugeneia/snabb,eugeneia/snabb,alexandergall/snabbswitch,wingo/snabbswitch,dpino/snabbswitch,Igalia/snabb,Igalia/snabbswitch,Igalia/snabb,Igalia/snabb,dpino/snabb