qemu: use consistent error when qemu binary is too old

qemu: use consistent error when qemu binary is too old

Most of our errors complaining about an inability to support a
particular action due to qemu limitations used CONFIG_UNSUPPORTED,
but we had a few outliers.  Reported by Jiri Denemark.

* src/qemu/qemu_command.c (qemuBuildDriveDevStr): Prefer
CONFIG_UNSUPPORTED.
* src/qemu/qemu_driver.c (qemuDomainReboot)
(qemuDomainBlockJobImpl): Likewise.
* src/qemu/qemu_hotplug.c (qemuDomainAttachPciControllerDevice):
Likewise.
* src/qemu/qemu_monitor.c (qemuMonitorTransaction)
(qemuMonitorBlockJob, qemuMonitorSystemWakeup): Likewise.