virttest.aexpect: Use /bin/bash instead of /bin/sh

Some particular Linux distrio, such as debian, uses a
shell program called dash. Unfortunately, the dash
program can't work with "dash -c source foobar" command.

Hence let's use '/bin/bash' explicitly, instead of the
ambiguous '/bin/sh' symbol link.
Signed-off-by: NTom Zhou <zhouqt@gmail.com>