qemu-option: Make qemu_opts_foreach() accumulate return values

Return the bitwise inclusive or of all return values instead of the
last call's value.  This lets you find out whether any of the calls
returned a non-zero value.

No functional change, as existing users either don't care for the
value, or pass non-zero abort_on_failure, which breaks the loop on the
first non-zero return value.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
