hw/mips/mips_malta: Create IDE hard drive array dynamically

hw/mips/mips_malta: Create IDE hard drive array dynamically

In the next commit we'll refactor the PIIX4 code out of
mips_malta_init(). As a preliminary step, add the 'ide_drives'
variable and create the drive array dynamically.

Reviewed-by: Aleksandar Markovic <[email protected]>
Reviewed-by: Li Qiang <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>