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

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

This reverts commit aad7b5c11d51d57659978e04702ac970906894e8.

The change from OvmfX64 to OvmfXen causes a change in behaviour, whereby
OvmfXen maps its shared info page at the top of address space.  When trying to
migrate such a domain, XENMEM_maximum_gpfn returns a very large value.  This
has uncovered multiple issues:

 1) The userspace hypercall wrappers truncate all return values to int on
    Linux and Solaris, even on 64bit.  This needs fixing in libxenctrl.
 2) 32bit toolstacks can't migrate any domain with RAM above the 2^40 mark,
    because of virtual address constraints.  This needs fixing in OVMF.

Fixes for both of these aren't completely trivial.  Revert the change to
unblock staging in the meantime.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>