kvmtool: Update documentation to mark version as stand-alone

At this point kvmtool compiles fines out of the Linux tree (at least
for x86 and arm/arm64), so update the README to reflect this.
Also add an INSTALL file to give more user friendly building
instructions.
Document the optional libraries and their common package names on the
way.

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