qemu.tests: fix issue abort can't find_pci device in guest

new device not always should in last line of lspci output, update
pci_find_cmd to reading new device from diff of lscpi output;

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: print device hotpulg/unplug repeat step info to log file

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #1374 from xutian/fix_block_hotplug

qemu.tests: fix issue abort can't find_pci device in guest