The kvmtool documentation is somewhat lacking, also it is not easily
accessible when living in the source tree only.
Add a good ol' manpage to document at least the basic commands and
their options.
This level of documentation matches the one that is already there in
the Documentation directory and should be subject to extension.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>