restapi: Enable support for Vm Boot menu

Exposed VmBase#bootMenuEnabled property as

  <bios>
    <boot_menu>
      <enabled>true|false</enabled>
    </boot_menu>
  </bios>

element of Vm and Template resources.
This property controls wheter to display menu upon system boot.
Default is false.

This schema also enables as to extend the bios element with further
parameters in the future as well as the boot menu (possibly adding
delay).

Change-Id: I926aaf58da72494f2e1710aec8530fe4794c2fe4
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1047624
Signed-off-by: Martin Betak <mbetak@redhat.com>