HMP: support specifying dump format for dump-guest-memory

HMP: support specifying dump format for dump-guest-memory

Dumping guest memory is available to specify the dump format now. This patch
adds options '-z|-l|-s' to HMP command dump-guest-memory to specify dumping in
kdump-compression format, with zlib/lzo/snappy compression. And without these
options ELF format will be used.

The discussion about this feature is here:

http://lists.nongnu.org/archive/html/qemu-devel/2014-03/msg04235.html

Signed-off-by: Qiao Nuohan <qiaonuohan@cn.fujitsu.com>
Suggested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com> (on s390x/kvm)
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>