[Libguestfs] v2v: Support conversion of EFI guests (RHBZ#1184690)

VMware allows you to present UEFI firmware to guests (instead of the ordinary PC BIOS). Virt-v2v can convert these guests, but requires that UEFI is supported by the target hypervisor. Currently KVM supports OVMF, a partially open source UEFI firmware, and can run these guests. Since OVMF support was only recently added to KVM (in 2014/2015), only output to raw qemu or libvirt is supported. Rich.