While reading Chris's code for fd migration I noticed the duplication
between QEMUFilePopen and QEMUFileStdio.  This fixes it, and makes
qemu_fopen more similar qemu_popen.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>