Fix the inconsistency in x509-dh-key-file parameter

Fix the inconsistency in x509-dh-key-file parameter

Commit c448e8552b0f6135dabddf944a7110f929c08320 (spice: tls
support) added options to the -spice command line. But there
is an inconsistency between the declaration of the option
'x509-dh-key-file' to -spice command line and its parameter
parsing 'x509-dh-file' in function qemu_spice_init.

https://bugs.launchpad.net/qemu/+bug/1035042

Reported-by: Alon Bar-Lev <alon.barlev@gmail.com>
Signed-off-by: Lei Li <lilei@linux.vnet.ibm.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>