OvmfPkg/OvmfXen: Build platform info HOB in XenPlatformPei

OvmfPkg/OvmfXen: Build platform info HOB in XenPlatformPei

Copy the function BuildPlatformInfoHob() from OvmfPkg/PlatformPei.

QemuFwCfgLib expect this HOB to be present, or fails to do anything.
InternalQemuFwCfgIsAvailable() from QemuFwCfgPeiLib module will not
check if the HOB is actually present for example and try to use a NULL
pointer.

Fixes: cda98df16228 ("OvmfPkg/QemuFwCfgLib: remove mQemuFwCfgSupported + mQemuFwCfgDmaSupported")
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>