x86/encrypted-state: Handle DR7 read/write events

Add code to handle #VC exceptions on DR7 register reads and writes. This
is needed early because show_regs() reads DR7 to print it out.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Co-developed-by: Joerg Roedel <jroedel@suse.de>
[ jroedel@suse.de: - Adapt to #VC handling framework
                   - Optimize register_from_idx()
                   - Support early usage ]
Signed-off-by: Joerg Roedel <jroedel@suse.de>