monitor: Fix crashes when using HMP commands without CPU

monitor: Fix crashes when using HMP commands without CPU

When running certain HMP commands ("info registers", "info cpustats",
"info tlb", "nmi", "memsave" or dumping virtual memory) with the "none"
machine, QEMU crashes with a segmentation fault. This happens because the
"none" machine does not have any CPUs by default, but these HMP commands
did not check for a valid CPU pointer yet. Add such checks now, so we get
an error message about the missing CPU instead.

Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <1484309555[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Acked-by: David Gibson <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>