qemu: unify code for reporting errors from QEMU log files
    
    There are two pretty similar functions qemuProcessReadLog and
    qemuProcessReadChildErrors. Both read from the QEMU log file
    and try to strip out libvirt messages. The latter then reports
    an error, while the former lets the callers report an error.
    
    Re-write qemuProcessReadLog so that it uses a single read
    into a dynamically allocated buffer. Then introduce a new
    qemuProcessReportLogError that calls qemuProcessReadLog
    and reports an error.
    
    Convert all callers to use qemuProcessReportLogError.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>