arch-arm: don't expose FEAT_VHE by default

arch-arm: don't expose FEAT_VHE by default

If FEAT_VHE is implemented and Linux boots in EL2, it programs itself
to operate in EL2. This causes a later boot stall as explained in
https://gem5.atlassian.net/browse/GEM5-901.
We provide a parameter "have_vhe" to enable FEAT_VHE on demand. This is
disabled by default until fixed. This avoids users stalling on the common
case of booting Linux without a hypervisor.

Change-Id: I3ee7be1ca59afc0cbbda59fb3aad4c897c06405f
Signed-off-by: Adrian Herrera <adrian.herrera@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39695
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>