KVM: selftests: Dedup vgic_init's asserts and improve error messages

Move the asserts for the many REDIST_REGS accesses into common helpers
instead of copy+pasting the same, unhelpful asserts over and over.  Not
providing the actual (or expected) value makes it unnecessarily painful
to debug failures, especially since test_assert() prints the errno
unconditionally, e.g. on success, it may print a stale, misleading errno.

Use kvm_device_attr_get() to handle the "success" check so that the
"success" and "expected == actual" asserts are separated, which will make
it far less likely that a user incorrectly assumes the ioctl() failed.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>