qemu_hotplug: Properly check for qemuMonitorDelDevice retval

qemu_hotplug: Properly check for qemuMonitorDelDevice retval

Luckily, the function returns only 0 or -1 so all the checks work
as expected. Anyway, our rule is that a positive value means
success so if the function ever returns a positive value these
checks will fail. Make them check for a negative value properly.

At the same time fix qemuDomainDetachExtensionDevice() reval
check. It is somewhat related to the aim of this patch.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>