virttest: Add force_drive_format for images device

Force guest to use specific drive format in command line. This can be
used for test some not default drive type like ide-drive in new version
of qemu-kvm.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1795 from ypu/force_drive_format

virttest: Add force_drive_format for images device