vstart.sh: enable creating multiple OSDs backed by spdk backend

vstart.sh: enable creating multiple OSDs backed by spdk backend

Currently vstart.sh only support deploying one OSD based on NVMe SSD.
The following two cases will cause errors:
1.There are 2 more NVMe SSDs from the same vendor on the machine
2.Trying to deploy 2 more OSDs if we only get 1 pci_id available

Add the support for allowing deploying multiple OSDs on a machine with
multiple NVME SSDs.

Change-Id: I6016435c1438bb4d16aff31f4575e03ccd3c9b3d

Signed-off-by: Richael Zhuang <richael.zhuang@arm.com>