exec: add parameter errp to gethugepagesize

exec: add parameter errp to gethugepagesize

Add parameter errp to gethugepagesize thus callers can handle errors.

If user adds a memory-backend-file object using object_add command,
specifying a non-existing directory for property mem-path, qemu will
core dump with message:

  /nonexistingdir: No such file or directory
  Bad ram offset fffffffffffff000
  Aborted (core dumped)

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

  qemu-system-x86_64: -object memory-backend-file,mem-path=/nonexistingdir,id=mem-file0,size=128M:
  failed to get page size of file /nonexistingdir: No such file or directory

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>