xen/arm64: config: Correctly define VMAP_VIRT_END

xen/arm64: config: Correctly define VMAP_VIRT_END

The vmap initialization code (vm_init_type) will round down
the end of the region to a page-aligned address.

On ARM64, the default vmap region is located between 1G and 2G.
Based on the initialization code, the end address is excluded
of the region.

Therefore the current definition of VMAP_VIRT_END will lead the
vmap code to not use the last 4K of the region.

Fix it by defining VMAP_VIRT_END as "VMAP_VIRT_START + GB(1)".

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>