qemu: Return meaningful error when qemu dies early

qemu: Return meaningful error when qemu dies early

https://bugzilla.redhat.com/show_bug.cgi?id=844378

When qemu dies early after connecting to its monitor but before we
actually try to read something from the monitor, we would just fail
domain start with useless message:

    "An error occurred, but the cause is unknown"

This is because the real error gets reported in a monitor EOF handler
executing within libvirt's event loop.

The fix is to take any error set in qemuMonitor structure and propagate
it into the thread-local error when qemuMonitorClose is called and no
thread-local error is set.

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