Split utils_test according sub virt type.

Hi all:

With 2000+ lines in utils_test, expanding utils_test will
be not convenient. And to support different types' test(libvirt,
qemu, libguestfs...), now splitting it according its type.
```
virttest----utils_test
         |
         \--utils_test_lib---general
                           |-qemu---utils_test
                           |-libvirt---utils_test
                           \-...
```
Just do "import virttest.utils_test" in testcase, then
it will import sub-utils_test according environment
SUB_VIRT_TYPE in control file. If SUB_VIRT_TYPE do not exist,
only general functions will be imported.