runqemu: add user mode (SLIRP) support to x86 QEMU targets

runqemu: add user mode (SLIRP) support to x86 QEMU targets

Using 'slirp' as a command line option to runqemu will start QEMU
with user mode networking instead of creating tun/tap devices.
SLIRP does not require root access. By default port 2222 on the
host will be mapped to port 22 in the guest. The default port
mapping can be overwritten with the QB_SLIRP_OPT variable e.g.

QB_SLIRP_OPT = "-net nic,model=e1000 -net user,hostfwd=tcp::2222-:22"

Signed-off-by: Todor Minchev <todor.minchev@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>