qemu.tests.qemu_guest_agent: Install dependent DLL files with script

The previous patches for adding windows support of qemu-guest-agent
depend on a guest image in which user has installed necessary DLL
files.

This patch tries to install these DLLs by test script, so that to
we can test this feature with a fresh windows system.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.cfg: Fix the config of qemu guest agent test

The previous patch introduced a bug in the qemu-ga.exe path.
This patch tries to fix this problem.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>