Quite a long time ago, (apparently between qemu 0.12 and 0.13) qemu
quietly began supporting the optional specification of a domain in the
host-side address of all pci passthrough commands (by simply
prepending it to the bus:slot.function format, as
"dddd:bb:ss.f"). Since machines with multiple PCI domains are very
rare, this never came up in practice, so libvirt was never updated to
support it.

This patch takes the first step to supporting specification of a non-0
domain in the host-side address of PCI devices being assigned to a
domain, by adding a capability bit to indicate support
"QEMU_CAPS_HOST_PCI_MULTIDOMAIN", and detect it. Since this support
was added in a version prior to the minimum version required for
QMP-style capabilities detection, the capability is always enabled for
any qemu that uses QMP for capabilities detection. For older qemus,
the only clue that a domain can be specified in the host pci address
is the presence of the string "[seg:]" in the help string for
-pcidevice. (Ironically, libvirt will not be modified to support
specification of domain for -pcidevice, since any qemu new enough for
us to care about also supports "-device pci-assign" or "-device
vfio-pci", which are greatly preferred).