qemu: support alternate path of qemu binaries

Support using an alternate QEMU path for when we want to use a qemu
version not available in the SDK.

To use the alternate qemu version, export QEMU_BIN_PATH and point it
to the bin directory which contains the qemu executables.

For example:

export QEMU_BIN_PATH=/usr/local/bin

Signed-off-by: Anas Nashif <anas.nashif@intel.com>