pc: memory: Validate alignment of maxram_size to page size

pc: memory: Validate alignment of maxram_size to page size

If the maxram_size is not aligned and dimm devices were added on the
command line qemu would terminate with a rather unhelpful message:

ERROR:hw/mem/pc-dimm.c:150:pc_dimm_get_free_addr: assertion failed:
(QEMU_ALIGN_UP(address_space_size, align) == address_space_size)

In case no dimm device was originally added on the commandline qemu
exits on the assertion failure.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>