x86 UEFI: Implement UEFI function calls

This commit implements helper functions that call UEFI services and
assist the boot up process.

Signed-off-by: Varad Gautam <varad.gautam@suse.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20211004204931.1537823-4-zxwang42@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>