fix: use virtiofs mount type for krunkit vm type

[bigbitbox]

Problem

colima start --vm-type krunkit (without an explicit --mount-type) fails to boot the VM:

Failed to set config  error="rpc error: code = Unknown desc = field `mountType` must be `virtiofs` or `reverse-sshfs` for krunkit driver, got `9p`"
level=fatal msg="failed to create driver instance: ... failed to set config"

Reproduced with colima v0.10.3 on macOS 26.3.2 (Apple Silicon), krunkit 1.3.2.

Cause

cmd/start.go already knows krunkit is a virtiofs VM type (it does not downgrade virtiofs to sshfs for krunkit, and it rewrites 9p to virtiofs), but environment/vm/lima/yaml.go resolves the Lima mount type as:

default:
	if l.VMType == limaconfig.VZ {
		l.MountType = limaconfig.VIRTIOFS
	} else { // qemu
		l.MountType = limaconfig.NINEP
	}

so for krunkit the mount type ends up as 9p, which the krunkit driver rejects.

Fix

Resolve virtiofs for krunkit the same way as for vz. qemu keeps using 9p.

Tests

Added Test_config_MountType covering krunkit/vz/qemu with virtiofs, 9p and sshfs.
Without the fix the new test fails with:

--- FAIL: Test_config_MountType/krunkit_virtiofs
    yaml_test.go:140: mount type = 9p, want virtiofs

Workaround until this is released: colima start --vm-type krunkit --mount-type sshfs.