OvmfPkg: Change License.txt from 2-Clause BSD to BSD+Patent

OvmfPkg: Change License.txt from 2-Clause BSD to BSD+Patent

Change contents of License.txt in OvmfPkg of the edk2 repository
from the 2-Clause BSD License to the BSD+Patent License for the
default license type.  Preserve the contents related to the use
of the MIT license in this file.

https://bugzilla.tianocore.org/show_bug.cgi?id=1373

This change is based on the following emails:

  https://lists.01.org/pipermail/edk2-devel/2019-February/036260.html
  https://lists.01.org/pipermail/edk2-devel/2018-October/030385.html

RFCs with detailed process for the license change:

  V3: https://lists.01.org/pipermail/edk2-devel/2019-March/038116.html
  V2: https://lists.01.org/pipermail/edk2-devel/2019-March/037669.html
  V1: https://lists.01.org/pipermail/edk2-devel/2019-March/037500.html

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>