tools/firmware/ovmf: Use OvmfXen platform file is exist

tools/firmware/ovmf: Use OvmfXen platform file is exist

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

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <iwj@xenproject.org>
(cherry picked from commit aad7b5c11d51d57659978e04702ac970906894e8)
(cherry picked from commit 7988ef515a5eabe74bb5468c8c692e03ee9db8bc)