virt: sev-guest: Mark driver struct with __refdata to prevent section mismatch

virt: sev-guest: Mark driver struct with __refdata to prevent section mismatch

As described in the added code comment, a reference to .exit.text is ok for
drivers registered via module_platform_driver_probe(). Make this explicit to
prevent the following section mismatch warning:

  WARNING: modpost: drivers/virt/coco/sev-guest/sev-guest: section mismatch in reference: \
    sev_guest_driver+0x10 (section: .data) -> sev_guest_remove (section: .exit.text)

that triggers on an allmodconfig W=1 build.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/4a81b0e87728a58904283e2d1f18f73abc69c2a1.1711748999.git.u.kleine-koenig@pengutronix.de