[As of 15-OCT-2015; Tested with version
qemu-system-x86-2.3.1-7.fc22.x86_64]

$ qemu-system-x86_64 -M \?
Supported machines are:
pc                   Standard PC (i440FX + PIIX, 1996) (alias of
pc-i440fx-2.3)
pc-i440fx-2.3        Standard PC (i440FX + PIIX, 1996) (default)
pc-i440fx-2.2        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-2.1        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-2.0        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-1.7        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-1.6        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-1.5        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-1.4        Standard PC (i440FX + PIIX, 1996)
pc-1.3               Standard PC (i440FX + PIIX, 1996)
pc-1.2               Standard PC (i440FX + PIIX, 1996)
pc-1.1               Standard PC (i440FX + PIIX, 1996)
pc-1.0               Standard PC (i440FX + PIIX, 1996)
pc-0.15              Standard PC (i440FX + PIIX, 1996)
pc-0.14              Standard PC (i440FX + PIIX, 1996)
pc-0.13              Standard PC (i440FX + PIIX, 1996)
pc-0.12              Standard PC (i440FX + PIIX, 1996)
pc-0.11              Standard PC (i440FX + PIIX, 1996)
pc-0.10              Standard PC (i440FX + PIIX, 1996)
q35                  Standard PC (Q35 + ICH9, 2009) (alias of
pc-q35-2.3)
pc-q35-2.3           Standard PC (Q35 + ICH9, 2009)
pc-q35-2.2           Standard PC (Q35 + ICH9, 2009)
pc-q35-2.1           Standard PC (Q35 + ICH9, 2009)
pc-q35-2.0           Standard PC (Q35 + ICH9, 2009)
pc-q35-1.7           Standard PC (Q35 + ICH9, 2009)
pc-q35-1.6           Standard PC (Q35 + ICH9, 2009)
pc-q35-1.5           Standard PC (Q35 + ICH9, 2009)
pc-q35-1.4           Standard PC (Q35 + ICH9, 2009)
isapc                ISA-only PC
none                 empty machine
xenfv                Xen Fully-virtualized PC
xenpv                Xen Para-virtualized PC