virt/sev-guest: Add a MODULE_ALIAS

Autoload the driver when, for example, SNP init code creates the
corresponding platform device.

  [ bp: Rewrite commit message. ]

Fixes: fce96cf0 ("virt: Add SEV-SNP guest driver")
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/ff480c5e688eb0a72a4db0a29c7b1bb54c45bfd4.1667594253.git.crobinso@redhat.com