arm: virt: Add an abstract ARM virt machine type

arm: virt: Add an abstract ARM virt machine type

In preparation for future ARM virt machine types, this patch creates
an abstract type for all ARM machines. The current machine type in
QEMU (i.e. "virt") is renamed to "virt-2.6", whose naming scheme is
similar to other architectures. For the purpose of backward compatibility,
"virt" is converted to an alias, pointing to "virt-2.6". With this patch,
"qemu -M ?" lists the following virtual machine types along with others:

virt                 QEMU 2.6 ARM Virtual Machine (alias of virt-2.6)
virt-2.6             QEMU 2.6 ARM Virtual Machine

Signed-off-by: Wei Huang <wei@redhat.com>
Message-id: 1457717778-17727-2-git-send-email-wei@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>