spice-core: Use g_strdup_printf instead of snprintf

Several places in spice-core.c were using either g_malloc+snprintf
or snprintf+g_strdup to achieve the same result as g_strdup_printf.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
