virttest: Extract output between last two prompts in shell session

In virsh_net_start case, occasionally net_state_dict can't find
default network in net-list command output from virsh session. The
cause is list command output is mixed into net-list command output
of unknown reason.

For solving the problem, make output of virsh session command only
contains content start from the second prompt if the second prompt
is found in output reversed list.

Signed-off-by: Wayne Sun <gsun@redhat.com>