Validate the hostname setting

plugins/kernel_v2/config/vm.rb

@@ -243,6 +243,8 @@ module VagrantPlugins
         errors << I18n.t("vagrant.config.vm.box_missing") if !box
         errors << I18n.t("vagrant.config.vm.box_not_found", :name => box) if \
           box && !box_url && !machine.box
+        errors << I18n.t("vagrant.config.vm.hostname_invalid_characters") if \
+          @hostname && @hostname !~ /^[-.a-z0-9]+$/i
 
         has_nfs = false
         @synced_folders.each do |id, options|

templates/locales/en.yml

@@ -425,6 +425,9 @@ en:
         base_mac_invalid: "Base MAC address for eth0/NAT must be set. Contact box maintainer for more information."
         box_missing: "A box must be specified."
         box_not_found: "The box '%{name}' could not be found."
+        hostname_invalid_characters: |-
+          The hostname set for the VM should only contain letters, numbers,
+          and hyphens.
         network_invalid: |-
           The network type '%{type}' is not valid. Please use
           'hostonly' or 'bridged'.