ide: Add "model=s" qdev option

Allow the user to override the default disk model name "QEMU HARDDISK".

Some Linux distributions use the /dev/disk/by-id/scsi-SATA_name-of-disk-
model_serial addressing scheme when refering to partitions in /etc/fstab
and elsewhere. This causes problems when starting a disk image taken from
an existing physical server under qemu, because when running under qemu
name-of-disk-model is always "QEMU HARDDISK".

This patch introduces a model=s option which in combination with the
existing serial=s option can be used to fake the disk the operating
system was previously on, allowing the OS to boot properly.

Cc: kwolf@redhat.com
Signed-off-by: Floris Bos <dev@noc-ps.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
