lxc: Fix a bug of baselineCPU parse failure

When starting nova-compute on compute node with lxc hypervisor,
a valueError exception will be raised because a libvirt bug of
the incorrect return value of baselineCPU() function, in function
get_host_capabilities, which introduced by commit
3b8c26321bce07b8ad588099c1fdcef858e748a8.

For libvirt interface baselineCPU() in python library, -1 will be
returned if basecpu info is None from version 1.1.2, and the return
value of this interface is corrected to None from version 1.2.0.

So a checking is added to make sure the return value of baselineCPU()
is not -1, and also a test case is added.

Closes-bug: #1259796

Co-authored-by: Wangpan <hzwangpan@corp.netease.com>

Change-Id: I5e4cb4a8d09b660edc4f6c6a4ad4bfe58a4dd5cb
Merge "lxc: Fix a bug of baselineCPU parse failure"