Allow QEMUMachine to override reset sequencing

qemu_system_reset() function always performs the same basic actions on
all machines.  This includes running all the reset handler hooks,
however the order in which these will run is not always easily predictable.

This patch splits the core of qemu_system_reset() - the invocation of
the reset handlers - out into a new qemu_devices_reset() function.
qemu_system_reset() will usually call qemu_devices_reset(), but that
can be now overriden by a new reset method in the QEMUMachine
structure.

Individual machines can use this reset method, if necessary, to
perform any extra, machine specific initializations which have to
occur before or after the bulk of the reset handlers.  It's expected
that the method will call qemu_devices_reset() at some point, but if
the machine has really strange ordering requirements between devices
resets it could even override that with it's own reset sequence (with
great care, obviously).

For a specific example of when this might be needed: a number of
machines (but not PC) load images specified with -kernel or -initrd
directly into the machine RAM before booting the guest.  This mostly
works at the moment, but to make this actually safe requires that this
load occurs after peripheral devices are reset - otherwise they could
have active DMAs in progress which would clobber the in memory images.
Some machines (notably pseries) also have other entry conditions which
need to be set up as the last thing before executing in guest space -
some of this could be considered "emulated firmware" in the sense that
the actions of the firmware are emulated directly by qemu rather than
by executing a firmware image within the guest.  When the platform's
firmware to OS interface is sufficiently well specified, this saves
time both in implementing the "firmware" and executing it.

aliguori: don't unconditionally dereference current_machine

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
