OvmfPkg/XenResetVector: Silent a warning from nasm

To avoid nasm generating a warning, replace the macro by the value
expected to be stored in eax.
  Ia32/XenPVHMain.asm:76: warning: dword data exceeds bounds

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210412133003.146438-2-anthony.perard@citrix.com>