pvh: load initrd from fw_cfg

If we found initrd through fw_cfg, we can load it and use the
first module of hvm_start_info to pass initrd address and size
to the kernel.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>