Ensure boot order is preserved unless overridden (#1560)

Add boot_order to the merge function to ensure that provider config
sections will persist the value if defined in earlier blocks. However to
ensure it is possible to override, definitions in latter sections will
replace the earlier definitions, but only if provided.

Fixes: #937

lib/vagrant-libvirt/config.rb

@@ -1089,6 +1089,8 @@ module VagrantPlugins
 
       def merge(other)
         super.tap do |result|
+          result.boot_order = other.boot_order != [] ? other.boot_order : boot_order
+
           c = disks.dup
           c += other.disks
           result.disks = c