libxl: specify disks using supported command line syntax for new qemu

libxl: specify disks using supported command line syntax for new qemu

The -hdX syntax is only retained for compatibility reasons and the
-sdX syntax doesn't even exist.

Additionally convert the first four non-SCSI disks to hd[a-d] and
ignore any further non-SCSI disks (since qemu only supports 4 IDE
devices).

SCSI disks are passed through as is. qemu-xen was limited to 7 SCSI
devices but upstream qemu supports 256, therefore do not limit the
number of disks on the libxl side.

qemu-xen did all this itself internally.

Fixes "qemu: -xvda: invalid option" and allows PVHVM to work with
upstream qemu.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>