qemu.tests: Use preprocess_vm to create vm in system_reset_bootable

Use preprocess_vm to handle the vm creation in this case, otherwise
some nic device init in host will be skipped.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #91 from ypu/system_reset

qemu.tests: Use preprocess_vm to create vm in system_reset_bootable