spec: explicitly avoid bhyve on Linux
    
    Generally, we try to make the spec file tweakable via user
    variables, so that they can select a different subset of sub-rpms
    to build.  We also try to explicitly list all driver config
    options, rather than leaving the chance that the rpm build may be
    non-deterministic based on what the user had installed locally.
    But in the case of the recent bhyve hypervisor driver, there is
    no port of bhyve to Linux, so it is easier to just blindly
    disable it for now.  If someone ever does try to port bhyve to
    Fedora, we can make the spec file conditional at that point.
    
    * libvirt.spec.in (%configure): Don't try to build bhyve.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>