OvmfPkg: Introduce XenResetVector

OvmfPkg: Introduce XenResetVector

Introduce XenResetVector, a copy of OvmfPkg/ResetVector, with one
changes:
  - SEC_DEFAULT_CR0: enable cache (bit 30 or CD set to 0)

Xen copies the OVMF code to RAM, there is no need to disable cache.

This new module will later be modified to add a new entry point, more
detail in a following commit "OvmfPkg/XenResetVector: Add new entry point
for Xen PVH"

Value FILE_GUID of XenResetVector have not changed compare to ResetVector
because it is a special value (gEfiFirmwareVolumeTopFileGuid).

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-4-anthony.perard@citrix.com>