arm: qmp: add query-gic-capabilities interface

This patch add "query-gic-capabilities" but does not implement it. The
command is ARM-only. The command will return a list of GICCapability
structs that describes all GIC versions that current QEMU and system
support.

Libvirt is possibly the first consumer of this new command.

Before this patch, a libvirt user can successfully configure all kinds
of GIC devices for ARM guests, no matter whether current QEMU/kernel
supports them. If the specified GIC version/type is not supported, the
user will get an ambiguous "QEMU boot failure" error when trying to start
the VM. This is not user-friendly.

With this patch, libvirt should be able to query which type (and which
version) of GIC device is supported. Using this information, libvirt
can warn the user during configuration of guests when specified GIC
device type is not supported. Or better, we can just list those versions
that we support, and filter out the unsupported ones.

For example, if we got the query result:

{"return": [{"emulated": false, "version": 3, "kernel": true},
            {"emulated": true, "version": 2, "kernel": false}]}

then it means that we support emulated GIC version 2 using:

  qemu-system-aarch64 -M virt,accel=tcg,gic-version=2 ...

or KVM-accelerated GIC version 3 using:

  qemu-system-aarch64 -M virt,accel=kvm,gic-version=3 ...

If we specify other explicit GIC versions rather than the above, QEMU
will not be able to boot.

The community is working on a more generic way to query these kinds of
information about valid values of machine properties. However, due to
the importance of supporting this specific use case, weecided to first
implement this ad-hoc one; then when the generic method is ready, we
can move on to that one smoothly.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1458788142-17509-2-git-send-email-peterx@redhat.com
[PMM: tweaked commit message a bit; monitor.o is CONFIG_SOFTMMU only]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
