virQEMUBuildNetdevCommandlineFromJSON: Prepare for quirky 'guestfwd'

virQEMUBuildNetdevCommandlineFromJSON: Prepare for quirky 'guestfwd'

QEMU models guestfwd as:

 'guestfwd': [
                 { "str": "tcp:10.0.2.1:4600-chardev:charchannel0" },
                 { "str": "...."},
             ]

but the command line as:

guestfwd=tcp:10.0.2.1:4600-chardev:charchannel0,guestfwd=...

I guess the original idea was to make it extensible while not worrying
about adding another object for it. Either way it requires us to add yet
another JSON->cmdline convertor for arrays.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>