This update for ovmf fixes the following issues:

- Added ovmf-x86_64-sev flavor to X64 against AMD SEV (bsc#1232762):

    * Moved "-D SECURE_BOOT_ENABLE" from OVMF_FLAGS to EXTRA_FLAGS_X64,
      BUILD_OPTIONS_AA64 and BUILD_OPTIONS_RV64 because SEV can NOT work
      with secure boot.
    * Add "-D SECURE_BOOT_ENABLE" to BUILD_OPTIONS_X86 because the
      building option be removed from OVMF_FLAGS.
    * The ovmf-x86_64-sev-code.bin, ovmf-x86_64-sev-vars.bin and a unified
      image ovmf-x86_64-sev.bin can be used.

- Added 50-ovmf-x86_64-sev.json and 60-ovmf-x86_64-sev.json to descriptors.tar.xz for SEV flavor:
    * Removed features tag:

       + "acpi-s4", "acpi-s3", "requires-smm", "secure-boot", "enrolled-keys"

    * Added features tag:

       + "amd-sev", "amd-sev-es", "amd-sev-snp"

    * The 50-ovmf-x86_64-sev.json is for the ovmf-x86_64-sev.bin unified image
      which is stateless mode.
    * The 60-ovmf-x86_64-sev.json is for the ovmf-x86_64-sev-code/vars.bin.
      Please note that the -vars storage is non-secure because SEV does NOT
      support SMM (requires-smm).

- Removed "amd-sev" and "amd-sev-es" from descriptors/60-ovmf-x86_64.json
  and descriptors/60-ovmf-x86_64-2m.json.