memsave: Improve and disambiguate error message

When requesting a size which cannot be read, the error message shows
a different address which is misleading to the user and it looks like
something's wrong with the address parsing. This is because the input
@addr variable is incremented in the memory dumping loop:

(qemu) memsave 0xffffffff8418069c 0xb00000 mem
Invalid addr 0xffffffff849ffe9c specified

Fix that by saving the original address and size and use them in the
error message:

(qemu) memsave 0xffffffff8418069c 0xb00000 mem
Invalid addr 0xffffffff8418069c/size 11534336 specified

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
