Add to support another format of memory size

[kakkoko]

vagrant-mutate cannot convert some virtualbox images:

% vagrant mutate ubuntu/xenial64 bhyve
Converting ubuntu/xenial64 from virtualbox to bhyve.
    (100.00/100%)
/home/kakkoko/.vagrant.d/gems/2.4.4/gems/vagrant-mutate-1.2.0/lib/vagrant-mutate/box/virtualbox.rb:153:in `size_in_bytes': Unknown unit byte * 2^20 (ArgumentError)
        from /home/kakkoko/.vagrant.d/gems/2.4.4/gems/vagrant-mutate-1.2.0/lib/vagrant-mutate/box/virtualbox.rb:69:in `block in memory'
        from /usr/local/lib/ruby/2.4/rexml/element.rb:927:in `block in each'
        from /usr/local/lib/ruby/2.4/rexml/xpath.rb:68:in `each'
        from /usr/local/lib/ruby/2.4/rexml/xpath.rb:68:in `each'
        from /usr/local/lib/ruby/2.4/rexml/element.rb:927:in `each'
        from /home/kakkoko/.vagrant.d/gems/2.4.4/gems/vagrant-mutate-1.2.0/lib/vagrant-mutate/box/virtualbox.rb:67:in `memory'
        from /home/kakkoko/.vagrant.d/gems/2.4.4/gems/vagrant-mutate-1.2.0/lib/vagrant-mutate/converter/bhyve.rb:24:in `generate_vagrantfile'
        from /home/kakkoko/.vagrant.d/gems/2.4.4/gems/vagrant-mutate-1.2.0/lib/vagrant-mutate/converter/converter.rb:61:in `write_vagrantfile'
        from /home/kakkoko/.vagrant.d/gems/2.4.4/gems/vagrant-mutate-1.2.0/lib/vagrant-mutate/converter/converter.rb:42:in `convert'
        from /home/kakkoko/.vagrant.d/gems/2.4.4/gems/vagrant-mutate-1.2.0/lib/vagrant-mutate/mutate.rb:50:in `execute'
        from /usr/local/lib/ruby/gems/2.4/gems/vagrant-2.0.3/lib/vagrant/cli.rb:46:in `execute'
        from /usr/local/lib/ruby/gems/2.4/gems/vagrant-2.0.3/lib/vagrant/environment.rb:269:in `cli'
        from /usr/local/lib/ruby/gems/2.4/gems/vagrant-2.0.3/bin/vagrant:154:in `<top (required)>'
        from /usr/local/bin/vagrant:23:in `load'
        from /usr/local/bin/vagrant:23:in `<main>'

OVF(DMTF DSP0004(*1)) defines flexible format of programatic units.
(for instance, "byte * 2^10" == "kibibytes")
This commit supports three often-used representation:

• "byte * 2^10" (means "kibibytes")
• "byte * 2^20" (means "mebibytes")
• "byte * 2^30" (means "gibibytes")

(*1) http://www.dmtf.org/standards/published_documents/DSP0004_2.7.0.pdf