virt: Make test can create env file on its own directory

When I execute libguestfs tests, "libvirt/env" will be updated/created,
there is no env file created in "libguestfs/".So do this modification to
make env file can be created/updated in "libguestfs/" and other test
directory.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>