virttest.qemu_vm: Typo fix: tapfds -> vhostfds

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.qemu_vm.py: Correct parameter in add_option_rom(), help -> devices

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #979 from FengYang/update2

some typo and bug fix in next tree