xg_main: If XEN_DOMCTL_gdbsx_guestmemio fails then force error.

xg_main: If XEN_DOMCTL_gdbsx_guestmemio fails then force error.

Without this gdb does not report an error.

With this patch and using a 1G hvm domU:

(gdb) x/1xh 0x6ae9168b
0x6ae9168b:     Cannot access memory at address 0x6ae9168b

Drop output of iop->remain because it most likely will be zero.
This leads to a strange message:

ERROR: failed to read 0 bytes. errno:14 rc:-1

Add address to write error because it may be the only message
displayed.

Note: currently XEN_DOMCTL_gdbsx_guestmemio does not change 'iop' on
error and so iop->remain will be zero.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Acked-by: Mukesh Rathor <mukesh.rathor@oracle.com>