Change LibvirtDriver.capabilities to an instance variable
Change LibvirtDriver.capabilities to an instance variable

Since at least one libvirt capability is already dynamic due to
depending on support being present in QEMU and libvirt, change the
LibvirtDriver.capabilities dictionary from a class variable to an
instance variable.  This prevents the risk of one test setting a
capability which bleeds over into other tests, in the same way it was
already done in the PowerVM driver:

    https://github.com/openstack/nova-powervm/commit/cc7564549a90a59f2ef27cca14862c33b9a07c25

Change-Id: Ic4c16b23834b43a258be5073e32bcdb9c0421f02

mahak/nova,openstack/nova,rahulunair/nova,klmitch/nova,mahak/nova,mahak/nova,rahulunair/nova,klmitch/nova,klmitch/nova,openstack/nova,klmitch/nova,rahulunair/nova,openstack/nova