virt: sevguest: Change driver name to reflect generic SEV support

During patch review, it was decided the SNP guest driver name should not
be SEV-SNP specific, but should be generic for use with anything SEV.
However, this feedback was missed and the driver name, and many of the
driver functions and structures, are SEV-SNP name specific. Rename the
driver to "sev-guest" (to match the misc device that is created) and
update some of the function and structure names, too.

While in the file, adjust the one pr_err() message to be a dev_err()
message so that the message, if issued, uses the driver name.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/307710bb5515c9088a19fd0b930268c7300479b2.1650464054.git.thomas.lendacky@amd.com