virttest.utils_test: Add support of set multiple gluster disks

As virsh attach gluster disk not supported now, update the
set_vm_disk function to support set multiple gluster disk.

The function used to delete all disks and append the new disk, now
only delete the disk with the same target dev as set, which keep
other disks.

Signed-off-by: Wayne Sun <gsun@redhat.com>