tools/firmware/ovmf: Use OvmfXen platform file if exist and update OVMF

tools/firmware/ovmf: Use OvmfXen platform file if exist and update OVMF

A platform introduced in EDK II named OvmfXen is now the one to use for
Xen instead of OvmfX64. It comes with PVH support.

Also, the Xen support in OvmfX64 is deprecated,
    "deprecation notice: *dynamic* multi-VMM (QEMU vs. Xen) support in OvmfPkg"
    https://edk2.groups.io/g/devel/message/75498
and has been removed upstream.

We need to also update to a newer version of OVMF as OvmfXen in the
release "edk2-stable202105" doesn't work well with Xen, so we need the
fix b37cfdd28071 ("OvmfPkg/XenPlatformPei: Relocate shared_info page
mapping").

Also, don't set anymore the number of thread for parallel build when
building the newer platform, OvmfPkg/build.sh is now doing parallel
build by default.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <iwj@xenproject.org>