vhost: fix deadlock on IOTLB miss

vhost: fix deadlock on IOTLB miss

An optimization was done to only take the iotlb cache lock
once per packet burst instead of once per IOVA translation.

With this, IOTLB miss requests are sent to Qemu with the lock
held, which can cause a deadlock if the socket buffer is full,
and if Qemu is waiting for an IOTLB update to be done.

Holding the lock is not necessary when sending an IOTLB miss
request, as it is not manipulating the IOTLB cache list, which
the lock protects. Let's just release it while sending the
IOTLB miss.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>