loader.efi: only reduce the size of the staging area on Hyper-V

loader.efi: only reduce the size of the staging area on Hyper-V

Doing this on physical hosts turns out to be problematic, e.g. see comment
24 and 28 in https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=211746.

To fix the real underlying issue correctly & thoroughly, IMO we need
a relocatable kernel, but that would require a lot of complicated long
term work:  https://reviews.freebsd.org/D9686?id=25414#inline-56969

For now, let's only apply efi_verify_staging_size() to VMs running on
Hyper-V, and restore the old behavior on physical machines since that
has been working for people for a long period of time, though that's
potentially unsafe...

MFC after:	2 weeks
Sponsored by:	Microsoft