virttest.qemu_vm: Make boot and bootindex works correct

This patch will fix wrong boot setup when set 'boot = on' in cfg files
for cases need more than one disk.

When 'boot = on' set in cfg files, will first check if qemu support
bootindex. If it does, we will use bootindex which start from 0 and
will increase 1 for each image. You can also set the bootindex in cfg
files. If it doesn't, we will setup the first image to 'boot=on', other
images will set to 'boot=off' which is the limit of boot option in qemu.

Also remove the duplicate boot setup line in add_drive()

Signed-off-by: Yiqiao Pu <ypu@redhat.com>