vhost: add new ready status flag

vhost: add new ready status flag

This patch adds a new status flag indicating the Virtio device
is ready to operate.

This is required to be able to call rte_vhost_mtu_get() in the
.new_device() callback, as rte_vhost_mtu_get needs that the
negotiation is done, but it is too early to rely on running status
flag, which is set just after .new_device() returns.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>