xen/arm: p2m: Rework the context switch to another VTTBR in flush_tlb_domain

xen/arm: p2m: Rework the context switch to another VTTBR in flush_tlb_domain

The current implementation of flush_tlb_domain is relying on the domain
to have a single p2m. With the upcoming feature altp2m, a single domain
may have different p2m. So we would need to switch to the correct p2m in
order to flush the TLBs.

Rather than checking whether the domain is not the current domain, check
whether the VTTBR is different. The resulting assembly code is much
smaller: from 38 instructions (+ 2 functions call) to 22 instructions.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>