Fix 'virtual' fact on FreeBSD

We should not call Linux-specific code from a non-Linux system.  This
cross-platform utility class needs to skip Linux code when running on a
non-linux platform.

lib/facter/util/facts/posix/virtual_detector.rb

@@ -16,10 +16,14 @@ def platform
             private
 
             def check_docker_lxc
+              return nil unless const_defined?('Facter::Resolvers::Linux')
+
               Facter::Resolvers::Linux::Containers.resolve(:vm)
             end
 
             def check_gce
+              return nil unless const_defined?('Facter::Resolvers::Linux')
+
               bios_vendor = Facter::Resolvers::Linux::DmiBios.resolve(:bios_vendor)
               'gce' if bios_vendor&.include?('Google')
             end
@@ -61,6 +65,8 @@ def check_openbsd
             end
 
             def check_other_facts
+              return nil unless const_defined?('Facter::Resolvers::Linux')
+
               bios_vendor = Facter::Resolvers::Linux::DmiBios.resolve(:bios_vendor)
               return 'kvm' if bios_vendor&.include?('Amazon EC2')