fix #2566: increase scsi limit to 31

fix #2566: increase scsi limit to 31

to achieve this we have to add 3 new scsihw addresses since lsi
controllers can only hold 7 scsi drives

we go up to 31, since this is the limit for virtio-scsi-single devices
we have reserved (we can increase this in the future)

to make it more future proof, we add a new pci bridge under pci
bridge 1, so we have to adapt the bridge adding code (we did not
need this for q35 previously)

impact on live migration:
since on older versions of qemu-server we do not have those config
settings, there is no problem from old -> new

new->old is not supported anyway and this breaks so that
the vm crashes and loses the configs for scsi15-30
(same behaviour as e.g. with audio0 and migration from new->old)

tested with 31 scsi disk on
i440fx + virtio-scsi
i440fx + lsi
q35 + virtio-scsi
q35 + lsi
with ovmf + seabios

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>