qemu: Set arch to i686 if qemu-system-i386 is found

If we can't probe the architecture from QMP we parse the architecture
from the qemu binaries name. This results in the architecture being i386
instead of i686 which then results in QEMU_CAPS_PCI_MULTIBUS being unset
which gives a broken qemu command line.

This probably didn't show up earlier since most of the time there's also
a /usr/bin/qemu around which results in i686 capabilities.

src/qemu/qemu_capabilities.c

@@ -2055,6 +2055,8 @@ qemuCapsInitHelp(qemuCapsPtr caps)
         /* For historical compat we use 'itanium' as arch name */
         if (STREQ(tmp, "ia64"))
             tmp = "itanium";
+        else if (STREQ(tmp, "i386"))
+            tmp = "i686";
     } else {
         uname_normalize(&ut);
         tmp = ut.machine;