exec.c: workaround regression caused by alignment change in d2f39ad

exec.c: workaround regression caused by alignment change in d2f39ad

Commit d2f39ad "exec.c: Ensure right alignment also for file backed ram"
added an additional alignment requirement on the size of backend file
besides the previous page size. On x86, the alignment is changed from
4KB in QEMU 2.6 to 2MB in QEMU 2.7.

This change breaks certain usages in QEMU 2.7 on x86, e.g.
    -object memory-backend-file,id=mem1,mem-path=/tmp/,size=$SZ
    -device pc-dimm,id=dimm1,memdev=mem1
where $SZ is multiple of 4KB but not 2MB (e.g. 1023M). QEMU 2.7
reports the following error message and aborts:
qemu-system-x86_64: -device pc-dimm,memdev=mem1,id=nv1: backend memory size must be multiple of 0x200000

The same regression may also happen in other platforms as indicated by
Igor Mammedov. This change is however necessary for s390 according to
the commit message of d2f39ad, so we workaround the regression by taking
the change only on s390.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reported-by: "Xu, Anthony" <anthony.xu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>