qemu: Make QMP probing process reusable

qemu: Make QMP probing process reusable

The code that runs a new QEMU process to be used for probing
capabilities is separated into four reusable functions so that any code
that wants to probe a QEMU process may just follow a few simple steps:

    cmd = virQEMUCapsInitQMPCommandNew(...);
    virQEMUCapsInitQMPCommandRun(cmd);

    /* talk to the running QEMU process using its QMP monitor */

    if (reprobeIsRequired) {
        virQEMUCapsInitQMPCommandAbort(cmd, ...);
        virQEMUCapsInitQMPCommandRun(cmd);

        /* talk to the running QEMU process again */
    }

    virQEMUCapsInitQMPCommandFree(cmd);

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>