virttest.boot: Move USB related code out of boot.py

To keep the boot.py clean and simple to read, remove
usb related code from boot.py

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Qemu Test: Create a separated script for usb device

These code existed in boot.py before. But it's better
to put them in a new separated file.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virttest.usb_boot: make this case run on windows guest

The previous usb boot test only works on linux guest.
This patch updates them and make them work on windows guest.

Also changes:
 - Add error context to check usb device functions.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Qemu Test: Move usb.py to usb_storage.py

Since this usb.py script is used to test
usb-storage device, rename it to usb_storage.py.
Also, as usb_storage.cfg is already consolidated
in usb.cfg, delete it.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
usb: Consolidate usb_multi_disk into usb

Consolidate usb_multi_disk into the usb tests. Now
all usb related tests can be run using:

./run -t qemu --tests usb

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>