qemu: add support for dump-guest-core option

qemu: add support for dump-guest-core option

The "dump-guest-core' option is new option for the machine type
(-machine pc,dump-guest-core) that controls whether the guest memory
will be marked as dumpable.

While testing this, I've found out that the value for the '-M' options
is not parsed correctly when additional parameters are used. However,
when '-machine' is used for the same options, it gets parsed as
expected. That's why this patch also modifies the parsing and creating
of the command line, so both '-M' and '-machine' are recognized. In
QEMU's help there is only mention of the 'machine parameter now with
no sign of the older '-M'.