Add machine type aliases

Add machine type aliases

Add an 'alias' field to QEMUMachine and display it in the output of
'qemu -M ?' with an '(aliased to foo)' suffix.

Aliases can change targets in newer versions of qemu, so management tools
may choose canonicalize machine types to ensure that if a user chooses an
alias, that the actual machine type used will remain compatible in
future.

This is intended to mimic a symlink to a machine description file.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>