libxc: correct error message in xc_sr_common.c

libxc: correct error message in xc_sr_common.c

When the record length for sending the p2m frames in a migration
stream is too large, the issued error message is not very helpful:

xc: Record (0x00000003, x86 PV P2M frames) length 0x8 exceeds max
    (0x800000): Internal error

When printing the error use the size which was tested instead that of
the record header length.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>