KVM: x86: Mark GPRs dirty when written

When performing VMGEXIT processing for an SEV-ES guest, register values
will be synced between KVM and the GHCB. Prepare for detecting when a GPR
has been updated (marked dirty) in order to determine whether to sync the
register to the GHCB.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <7ca2a1cdb61456f2fe9c64193e34d601e395c133.1607620209.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>