qemu: Add a capability flag for -no-acpi

qemu: Add a capability flag for -no-acpi

Currently non-x86 guests must have <acpi/> defined in <features> to
prevent libvirt from running qemu with -no-acpi. Although it works, it
is a hack.

Instead add a capability flag which indicates whether qemu understands
the -no-acpi option. Use it to control whether libvirt emits -no-acpi.

Current versions of qemu always display -no-acpi in their help output,
so this patch has no effect. However the development version of qemu
has been modified such that -no-acpi is only displayed when it is
actually supported.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>