kvm tools: fix boot of guests with more than 4gb of ram

Commit "kvm tools: virtio: remove hardcoded assumptions
about guest page size" has introduced a bug that prevented
guests with more than 4gb of ram from booting.

The issue is that 'pfn' is a 32bit integer, so when multiplying
it by page size to get the actual page will cause an overflow if
the pfn referred to a memory area above 4gb.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>