Add "SSD emulation" property to IDE, SATA, and SCSI drives

Add "SSD emulation" property to IDE, SATA, and SCSI drives

When enabled, the "SSD emulation" feature exposes drives as
solid-state (rather than rotational) by enabling the new `ssd`
qm.conf flag [1] on the drives, which in turn sets QEMU's
`rotation_rate` property [2, 3] on the underlying devices.

This change also moves the "Discard" option out of the advanced
section and promotes it to the top of the dialog box, as proposed
by Thomas Lamprecht [4].

[1] https://git.proxmox.com/?p=qemu-server.git;a=commit;h=6c875f9f315f2dc7a0a3a988622c2efdf089b171
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1498042
[3] https://lists.gnu.org/archive/html/qemu-devel/2017-10/msg00698.html
[4] https://pve.proxmox.com/pipermail/pve-devel/2018-November/034327.html

Signed-off-by: Nick Chevsky <nchevsky@gmail.com>