efi_selftest: fix SetVirtualAddressMap unit test

We read the address map before assigning the memory for the pages that will
be mapped to virtual addresses. So these pages will overlap with the entry
for EFI_CONVENTIONAL_MEMORY. We have to ensure that every page is described
at most once in the map.

Remove EFI_CONVENTIONAL_MEMORY from the map that we pass to
SetVirtualAddressMap().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>