target-arm: Make the 64-bit version of VTCR do the migration

target-arm: Make the 64-bit version of VTCR do the migration

Move the ALIAS tag from VTCR_EL2 to VTCR so that we migrate the
64-bit version, as is usual. (This has no particular effect now
unless the guest wrote to the high RES0 bits of VTCR_EL2.)
Add a comment about why it's OK that we don't have the various
accessor functions that the EL1 TCR regdefs do.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Sergey Fedorov <[email protected]>
Message-id: 1459435778[email protected]