run_tests.sh: Add kvmtool support

Modify run_tests.sh to use kvmtool instead of qemu to run tests when
kvm-unit-tests has been configured with --target=kvmtool.

Example invocation:

$ ./configure --target=kvmtool
$ make clean && make
$ ./run_scripts.sh

A custom location for the kvmtool binary can be set using the environment
variable KVMTOOL:

$ KVMTOOL=/path/to/kvmtool/binary ./run_scripts.sh

Standalone test support is absent, but will be added in subsequent patches.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>