virttest.libvirt_storage_unittest: remove ModuleLoadCheckVirsh class.

Since we initialize a bogus_virsh in setup to provide a fake virsh for unittest,
then there is no need to skip if virsh command is not available.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #833 from yangdongsheng/libvirt_storage_unittest

virttest.libvirt_storage_unittest: remove ModuleLoadCheckVirsh class.