qemu.tests: enhance pci_test_cmd for block_hotplug testing

Add rescan and list disk steps; Do rescan to ensure diskpart read latest disk
configuration and it will avoid diskpart report
'The disk you specified is not valid.' error, list disk step will record disk
info before check disk drive;
New steps will helpful for debugging when test failed at pci device test step;

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #838 from xutian/fix_block_hotplug

Enhance pci_test_cmd for block_hotplug testing