vdsm: add support for TPM device passthrough

Libvirt has basic support for TPM passthrough, which allows
VM to be given access directly to host's TPM module.

Engine constraints: vm with assigned TPM cannot be allowed to
migrate, host cannot pass single TPM to multiple VMs using
passthrough protocol (currently only supported protocol
http://wiki.qemu.org/Features/TPM)

Change-Id: I53a77a0977d367d1066e85590dd35b18bb5fa32a
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20649
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>