To assist with debugging, and to exercise the KVM_{GET,SET}_ONE_REG
interface extensions for SVE, this patch adds code to dump the full
contents of the SVE and/or FPSIMD registers when triggered using
lkvm debug -d.

This is a giant ugly hack and also slow (even with hacks to reduce
the number of dprintf() calls).

Due to a lack of buffering on one or both ends of the guest socket
connection, dumping the vcpu that's running on the same CPU as lkvm
debug seems to cause a lot of context switch thrashing also, but no
attempt has been made to address this here, since this patch is
in any case rather abusing the guest socket.  If we want to do
something like this for real, some more careful thought about how
to optimise the protocol is needed.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>