virt: move disk tests into a sub-directory

The test_virt_disk.py, test_virt_disk_vfs_guestfs.py, and
test_virt_disk_vfs_localfs.py test cases were in the top
level nova/tests/virt directory. Current policy is to use
a directory location that matches the source being tested,
so they should be in either nova/tests/virt/disk or
nova/tests/virt/disk/vfs respectively. The test_loop.py
and test_nbd.py tests should be in nova/tests/virt/disk/mount
too. Finally fakeguestfs.py should also be kept in the
nova/tests/virt/disk/vfs directory

Change-Id: Iebee539d202d5122939ae478fd482f19b19f604d
Merge "virt: move disk tests into a sub-directory"