KVM: selftests: x86: sync_regs_test: Get regs structure before modifying it

KVM: selftests: x86: sync_regs_test: Get regs structure before modifying it

The regs structure just accidentally contains the right values
from the previous test in the spot where we want to change rbx.
It's cleaner if we properly initialize the structure here before
using it.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20240208204844.119326-3-thuth@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>