We can let libvirt/QEMU figure out the best value to use.
Cache has always been set to 'default' anyway, and IO is set to
'threads', I don't think tweaking it makes a significant difference in
CRC's usecases (?)

This adds the required libvirt XML in order to share the host directories
listed in SharedDirs.
This will be made conditional on virtiofs support availability in the
next commits.

This adds a getBestGuestFromCaps helper which will be reused in the next
commits.

On older systems (RHEL7), virtiofs is not necessarily present.
This commit uses virsh domcapabilities to detect if virtiofs is
supported, and returns an error if not.

since IO/Cache is not set in DomainDiskDriver anymore
those are removed from the expected domain xml

the unit test TestTemplating is failing because the virtiofsSupported
check depends on a having a connection to an actual libvirt daemon
which is not present while running the unit tests

as a workaround, since we are not testing the virtiofs functionality
in unit tests, we call virtiofsSupported only when shared directories
are requested

this fixes the following error in test:
```
--- FAIL: TestTemplating (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x542294]

goroutine 20 [running]:
testing.tRunner.func1.2({0x65f1a0, 0xa269d0})
	/usr/lib/golang/src/testing/testing.go:1389 +0x24e
testing.tRunner.func1()
	/usr/lib/golang/src/testing/testing.go:1392 +0x39f
panic({0x65f1a0, 0xa269d0})
	/usr/lib/golang/src/runtime/panic.go:838 +0x207
github.com/libvirt/libvirt-go.(*Connect).GetCapabilities.func1(0x7f3a0c5f1fff?)
	/go/src/github.com/code-ready/machine-driver-libvirt/vendor/github.com/libvirt/libvirt-go/connect.go:493 +0x14
github.com/libvirt/libvirt-go.(*Connect).GetCapabilities(0x7f3a0a95bb00?)
	/go/src/github.com/code-ready/machine-driver-libvirt/vendor/github.com/libvirt/libvirt-go/connect.go:493 +0x1d
github.com/code-ready/machine-driver-libvirt/pkg/libvirt.getBestGuestFromCaps(0x7f3a3c21c758?)
	/go/src/github.com/code-ready/machine-driver-libvirt/pkg/libvirt/libvirt.go:268 +0x2b
github.com/code-ready/machine-driver-libvirt/pkg/libvirt.virtiofsSupported(0xc000040ef0?)
	/go/src/github.com/code-ready/machine-driver-libvirt/pkg/libvirt/domain.go:162 +0x30
github.com/code-ready/machine-driver-libvirt/pkg/libvirt.domainXML(0xc0000e3ef0, {0x6b1498, 0x3})
	/go/src/github.com/code-ready/machine-driver-libvirt/pkg/libvirt/domain.go:122 +0x9dc
github.com/code-ready/machine-driver-libvirt/pkg/libvirt.TestTemplating(0xc000087ba0?)
	/go/src/github.com/code-ready/machine-driver-libvirt/pkg/libvirt/domain_test.go:12 +0x1b9
testing.tRunner(0xc000126000, 0x6c8730)
	/usr/lib/golang/src/testing/testing.go:1439 +0x102
created by testing.(*T).Run
	/usr/lib/golang/src/testing/testing.go:1486 +0x35f
FAIL	github.com/code-ready/machine-driver-libvirt/pkg/libvirt	0.012s
FAIL
```