migration: setup bi-directional I/O channel for exec: protocol

migration: setup bi-directional I/O channel for exec: protocol

Historically the migration data channel has only needed to be
unidirectional. Thus the 'exec:' protocol was requesting an
I/O channel with O_RDONLY on incoming side, and O_WRONLY on
the outgoing side.

This is fine for classic migration, but if you then try to run
TLS over it, this fails because the TLS handshake requires a
bi-directional channel.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
(cherry picked from commit 062d81f0e968fe1597474735f3ea038065027372)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>