qemu: Add support for iothreads

qemu: Add support for iothreads

Add a new capability to ensure the iothreads feature exists for the qemu
emulator being run - requires the "query-iothreads" QMP command. Using the
domain XML add correspoding command argument in order to generate the
threads. The iothreads will use a name space "iothread#" where, the
future patch to add support for using an iothread to a disk definition to
merely define which of the available threads to use.

Add tests to ensure the xml/argv processing is correct.  Note that no
change was made to qemuargv2xmltest.c as processing the -object element
would require knowing more than just iothreads.