dump-guest-memory: more descriptive lookup_type failure

dump-guest-memory: more descriptive lookup_type failure

We've seen a few reports of

 (gdb) source /usr/share/qemu-kvm/dump-guest-memory.py
 Traceback (most recent call last):
   File "/usr/share/qemu-kvm/dump-guest-memory.py", line 19, in <module>
     UINTPTR_T = gdb.lookup_type("uintptr_t")
 gdb.error: No type named uintptr_t.

This occurs when symbols haven't been loaded first, i.e. neither a
QEMU binary was loaded nor a QEMU process was attached first. Let's
better inform the user of how to fix the issue themselves in order
to avoid more reports.

Acked-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20180314153820.18426-1-drjones@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>