introduce a new monitor command 'dump-guest-memory' to dump guest's memory

The command's usage:
   dump-guest-memory [-p] protocol [begin] [length]
The supported protocol can be file or fd:
1. file: the protocol starts with "file:", and the following string is
   the file's path.
2. fd: the protocol starts with "fd:", and the following string is the
   fd's name.

Note:
  1. If you want to use gdb to process the core, please specify -p option.
     The reason why the -p option is not default is:
       a. guest machine in a catastrophic state can have corrupted memory,
          which we cannot trust.
       b. The guest machine can be in read-mode even if paging is enabled.
          For example: the guest machine uses ACPI to sleep, and ACPI sleep
          state goes in real-mode.
  2. If you don't want to dump all guest's memory, please specify the start
     physical address and the length.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>