Introduce -machine command option.

Introduce -machine command option.

This option gives the ability to switch one "accelerator" like kvm, xen
or the default one tcg. We can specify more than one accelerator by
separate them by a colon. QEMU will try each one and use the first whose
works.

So,
./qemu -machine accel=xen:kvm:tcg

which would try Xen support first, then KVM and finally TCG if none of
the other works.

By default, QEMU will use TCG. But we can specify another default in the
global configuration file.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Alexander Graf <agraf@suse.de>