fsdev-proxy-helper: avoid TOC/TOU race

There is a minor time of check/time of use race between statfs and chroot.
It can be fixed easily by stat-ing the root after it has been changed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>