vTPM/TPM2: Support 'tpm2' extra command line.

vTPM/TPM2: Support 'tpm2' extra command line.

Make vtpm-stubdom domain compatible to launch on TPM 1.x / TPM 2.0.
Add:
..
     extra="tpm2=1"
..
to launch vtpm-stubdom domain on TPM 2.0, ignore it on TPM 1.x. for
example,
vtpm-stubdom domain configuration on TPM 2.0:

  kernel="/usr/lib/xen/boot/vtpmmgr-stubdom.gz"
  memory=16
  disk=["file:/var/scale/vdisk/vmgr,hda,w"]
  name="vtpmmgr"
  iomem=["fed40,5"]
  extra="tpm2=1"

vtpm-stubdom domain configuration on TPM 1.x:

  kernel="/usr/lib/xen/boot/vtpmmgr-stubdom.gz"
  memory=16
  disk=["file:/var/scale/vdisk/vmgr,hda,w"]
  name="vtpmmgr"
  iomem=["fed40,5"]

Signed-off-by: Quan Xu <quan.xu@intel.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>