KVM: SVM: Serialize access to the SEV ASID bitmap

The SEV ASID bitmap currently is not protected against parallel SEV guest
startups. This can result in an SEV guest failing to start because another
SEV guest could have been assigned the same ASID value. Use a mutex to
serialize access to the SEV ASID bitmap.

Fixes: 1654efcb

 ("KVM: SVM: Add KVM_SEV_INIT command")
Tested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>