qemu: monitor: Fix type of holdtime argument in qemuMonitorJSONSendKey

qemu: monitor: Fix type of holdtime argument in qemuMonitorJSONSendKey

qemuMonitorJSONSendKey declares the "holdtime" argument as unsigned int
while the command was constructed in qemuMonitorJSONMakeCommand using
the "P" modifier which took a unsigned long from the variable
arguments which then made it possible to access uninitialized memory.

This broke the qemumonitorjsontest on 32bit fedora 20:
64) qemuMonitorJSONSendKey
... libvirt: QEMU Driver error : internal error: unsupported data type 'W' for arg 'WVS\83ì \8bD$0è\91wÿÿ\81ÃAå' FAILED

Uncovered by upstream commit f744b831c66d9e82453f7a96cab5eddf7570c253.

Additionally add test for the hold-time option.