make the path of qemu binary specifiable

Current vagrant-kvm searches the qemu binary in a fixed manner
(as_libirt). This commit makes it specifiable via Vagrantfile like
this:

  config.vm.provider :kvm do |kvm|
	kvm.qemu_bin = "/custom/path/to/qemu-system-x86_64"
  end

Example error messages:

* User specified the binary, but it doesn't exist.
$  sudo vagrant up --provider=kvm
Bringing machine 'default' up with 'kvm' provider...
[default] Importing base box 'base'...
    (100.00/100%)
Executable binary of qemu could not be found. Please re-examine Vagrantfile (specified binary: asdf).

* Vagrant couldn't find the default installation of QEMU.
$ sudo vagrant up --provider=kvm
[default] Importing base box 'base'...
    (100.00/100%)
Executable binary of qemu could not be found. Please re-examine QEMU installation.