qemu: command: add support for acpi-bridge-hotplug feature

qemu: command: add support for acpi-bridge-hotplug feature

This change adds backend qemu command line support for new libvirt
global feature 'acpi-bridge-hotplug'. This option can be used as
following:

<feature>
  <pci>
    <acpi-bridge-hotplug state='off|on'/>
  </pci>
</feature>

The '<pci>' sub-element under '<feature>' is also newly introduced.

'acpi-bridge-hotplug' turns on the following command line option to
qemu for x86 guests:

(pc): -global PIIX4_PM.acpi-pci-hotplug-with-bridge-support=<off|on>

(q35): -global ICH9-LPC.acpi-pci-hotplug-with-bridge-support=<off|on>

This change also adds the required qemuxml2argv unit tests in order to
test correct qemu arguments. Unit tests have also been added to test
qemu capability validation checks as well as checks for using this
option with the right architecture.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
Reviewed-by: Laine Stump <laine@redhat.com>