multiboot: set boot_device to first partition

The multiboot info struct's 'boot_device' field has 'part1' set to 0x01, which
maps to the second primary partition. To specify the first primary partition,
'part1' should be set to 0x00, since partition numbers start from zero
according to the multiboot spec.

Signed-off-by: Arun Thomas <arun.thomas@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>