libvirt: add a test to guard against set_host_enabled raising an error

Commit cc70e3a adds a finally block to _get_new_connection to
automatically disable the host if making a connection to libvirt fails.

The set_host_enabled method currently swallows all exceptions which is
good so it doesn't mask the original libvirt failure in
_get_new_connection, but we need to have a test that makes sure that
isn't unintentionally regressed. This patch adds the test.

Related-Bug: #1257644

Change-Id: I4beec053794edc75ef2d0060a05285f7ed0b99ac
Merge "libvirt: add a test to guard against set_host_enabled raising an error"