efivar: Try harder to find label's efimedia

If there's no efimedia attribute on the provider, and the provider's a
glabel, then find the 'parent' geom. In this case, the provider's name
is label-type/name, but the geom's label will that of the underlying
device (eg ada0p1). If it is, recurisvely call find_geom_efimedia with
the geom's name, which shuold have the efimedia attribute.

Sponsored by: Netflix
Reviewed by: corvink, manu, asomers
Differential Revision: https://reviews.freebsd.org/D38614