cmd: nvedit_efi: use efi_auth_var_get_guid() for the default GUID

cmd: nvedit_efi: use efi_auth_var_get_guid() for the default GUID

do_env_set_efi() hand rolls the mapping from well known variable names
to their default vendor GUID and has already drifted from the canonical
name_type[] table in efi_var_common.c: it does not know "dbr", so
"env set -e dbr" operates on the variable under EFI_GLOBAL_VARIABLE_GUID
instead of the image security database GUID, silently creating a
variable that nothing will ever consume.

Convert the variable name to UTF-16 before selecting the GUID and let
efi_auth_var_get_guid() do the lookup. That function knows all
authenticated variables including "dbr" and falls back to
EFI_GLOBAL_VARIABLE_GUID for any other name, so behaviour is unchanged
for "db", "dbx", "dbt" and non-authenticated variables. Future
additions to the table now apply to the shell command automatically.

Since the conversion now happens before the value parsing loop, free
the UTF-16 name at the common exit label so the error path there does
not leak it.

Also drop the unmap_sysmem() call right after efi_set_variable_int():
the common exit path already unmaps the value for the -i case and frees
it otherwise, so the value was unmapped twice. On sandbox the second
call triggers a spurious "Address not mapped" warning for addresses
that get a tagged mapping.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>