virttest.virsh: Add a parameter for VirshSession to make
remote session clear.

With original logic of VirshSession.__init__(), we have
not choice but configuring auto-ssh-login for "local to
remote: virsh -c uri".
While for other types driver like esx, it will fail.
Now support login with password if auto-ssh-login is impossible.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>