spec: explicitly avoid bhyve on Linux

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>