x86 UEFI: introduce efi_exit

The combination of exit() + efi_rs_call() appears twice in the code,
unify them.

Signed-off-by: Zixuan Wang <zxwang42@gmail.com>
Message-Id: <20211116204053.220523-7-zxwang42@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>