virttest.env_process : add pci msi on/off support in autotest

Using this patch you can enable or disable pci msi by modify the
guest disk directly before test start.
if you want to disable (pci=nomsi) pci msi before test, just set:
    pci_msi_sensitive = yes
    disable_pci_msi = yes
in tests.cfg.

Install python-libguestfs before using this patch.

chang_log:
    V2 from V1:
        using python guestfs lib.
    V3 from v2:
        remove the env setting, make case more independent
        modify the default configure