spec: default vbox according to libvirtd build

spec: default vbox according to libvirtd build

Commit ba5f3c7 moved virtualBox support into libvirtd, but the spec
file was still unconditionally requesting it even when not building
the server side.  Thankfully there were no ill effects for a
client_only build, as most uses of %{with_vbox} were guarded by
%{with_libvirtd}; but we might as well avoid confusion by more
closely matching the makefile.

* libvirt.spec.in (with_vbox): Hoist to server conditionals.

Signed-off-by: Eric Blake <eblake@redhat.com>