Optimize machine option to set more options with it

Optimize machine option to set more options with it

Currently, -machine option is used only when dump-guest-core is set.

To use options defined in machine option for newer version of QEMU,
it needs to use -machine xxx, and to be compatible with older version
-M, this patch adds QEMU_CAPS_MACHINE_OPT capability for newer
version which supports -machine option.

Signed-off-by: Li Zhang <zhlcindy@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>