blkdeviotune: check for overflow when parsing XML

blkdeviotune: check for overflow when parsing XML

According to docs/schemas/domaincommon.rng and _virDomainBlockIoTuneInfo
all the iotune values are interpreted as unsigned long long, however
according to qemu_monitor_json.c, qemu silently truncates numbers
larger than LLONG_MAX. There's really not much of a usage for such
large numbers anyway yet. This patch provides the same overflow
check during a domain start as it does during setting
a blkdeviotune element in qemu_driver.c and thus reports an error when
a larger number than LLONG_MAX is detected.

https://bugzilla.redhat.com/show_bug.cgi?id=1131876