Enable "Discard" option for IDE and SATA drives

Enable "Discard" option for IDE and SATA drives

Even though QEMU supports the discard feature for both ATA [1] and
SCSI drives, the "Discard" checkbox in Proxmox VE is artificially
restricted to SCSI drives. This change expands availability of the
"Discard" checkbox to all drive types supported by QEMU, leaving
VirtIO Block as the only remaining exclusion.

Combined with the new "SSD emulation" option [2], enabling discard
on IDE/SATA drives allows reclaiming of free space on thin-provisioned
storage with guests that do not support our SCSI controllers.

[1] https://github.com/qemu/qemu/commit/d353fb72f59cd0e1f67baf773e74719cda761a89
[2] https://git.proxmox.com/?p=pve-manager.git;a=commit;h=f9261fde2134cec35a5ec7c3ebbee5daa4d67ec7

Signed-off-by: Nick Chevsky <nchevsky@gmail.com>
Tested-by: Thomas Lamprecht <t.lamprecht@proxmox.com>