exec: report error when memory < hpagesize

exec: report error when memory < hpagesize

Report an error when memory < hpagesize in file_ram_alloc() so callers
can handle the error.

If user adds a memory-backend-file object using object_add command,
specifying a size that is less than huge page size, qemu will core dump
with message:

  Bad ram offset fffffffffffff000
  Aborted (core dumped)

This patch fixes the problem. With this patch, qemu reports error
message like:

  qemu-system-x86_64: -object memory-backend-file,mem-path=/hugepages,id=mem-file0,size=1M: memory
  size 0x100000 must be equal to or larger than huge page size 0x200000

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>