x86: Consistently use safe_halt() in place of inline assembly

The safe_halt() library function simply performs an 'sti; hlt'
instruction sequence. There are several places where bare inline
assembly was used instead of this helper. Replace all open-coded
implementations with the helper.

No functional change intended.

Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20211019225351.970397-1-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>