Qemu before version 2.11 does not implement the architecture correctly,
and does not allow for a mask size of size different than 4.

This patch introduces a compatibility mode for such systems, forcing
the mask sizes to 4.

Since the mask size is currently still 4 anyway, this patch should have
no impact whatsoever by itself, but it will be needed when the mask size
is increased to 64 bits in the next patch.
Reviewed-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NClaudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>