qemu: simplify json parsing

Rather than grabbing an arbitrary JSON value and then checking
if it has the right type, we might as well request the correct
type to begin with.

* src/qemu/qemu_monitor_json.c (qemuMonitorJSONIOProcessEvent)
(qemuMonitorJSONCommandWithFd, qemuMonitorJSONHandleGraphics)
(qemuMonitorJSONGetStatus, qemuMonitorJSONExtractCPUInfo)
(qemuMonitorJSONGetVirtType, qemuMonitorJSONGetBalloonInfo)
(qemuMonitorJSONGetMemoryStats)
(qemuMonitorJSONDevGetBlockExtent)
(qemuMonitorJSONGetOneBlockStatsInfo)
(qemuMonitorJSONGetAllBlockStatsInfo)
(qemuMonitorJSONBlockStatsUpdateCapacityOne)
(qemuMonitorJSONBlockStatsUpdateCapacity)
(qemuMonitorJSONGetBlockExtent)
(qemuMonitorJSONGetMigrationStatusReply)
(qemuMonitorJSONGetDumpGuestMemoryCapability)
(qemuMonitorJSONAddFd, qemuMonitorJSONQueryRxFilterParse)
(qemuMonitorJSONExtractChardevInfo)
(qemuMonitorJSONDiskNameLookupOne)
(qemuMonitorJSONDiskNameLookup)
(qemuMonitorJSONGetAllBlockJobInfo)
(qemuMonitorJSONBlockIoThrottleInfo, qemuMonitorJSONGetVersion)
(qemuMonitorJSONGetMachines, qemuMonitorJSONGetCPUDefinitions)
(qemuMonitorJSONGetCommands, qemuMonitorJSONGetEvents)
(qemuMonitorJSONGetKVMState, qemuMonitorJSONGetObjectTypes)
(qemuMonitorJSONGetObjectListPaths)
(qemuMonitorJSONGetObjectProps, qemuMonitorJSONGetTargetArch)
(qemuMonitorJSONGetMigrationCapabilities)
(qemuMonitorJSONGetStringArray, qemuMonitorJSONAttachCharDev)
(qemuMonitorJSONGetCPUx86Data, qemuMonitorJSONGetIOThreads)
(qemuMonitorJSONGetMemoryDeviceInfo): Use shorter idioms.
Signed-off-by: NEric Blake <eblake@redhat.com>