target-ppc: dump-guest-memory support

This patch add support for dumping guest memory using dump-guest-memory
monitor command.

Before patch:

(qemu) dump-guest-memory testcrash
this feature or command is not currently supported
(qemu)

After patch:

(qemu) dump-guest-memory testcrash
(qemu)

crash was able to read the file

crash> bt
PID: 0      TASK: c000000000c0d0d0  CPU: 0   COMMAND: "swapper/0"

 R0:  0000000028000084    R1:  c000000000cafa50    R2:  c000000000cb05b0
 R3:  0000000000000000    R4:  c000000000bc4cb0    R5:  0000000000000000
 R6:  001efe93b8000000    R7:  0000000000000000    R8:  0000000000000000
 R9:  b000000000001032    R10: 0000000000000001    R11: 0001eb2117e00d55
....
...

NOTE: Currently crash tools doesn't look at ELF notes in the dump on ppc64.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
