Make disk type an optional parameter on get_disk_xml()

There are cases in which the disk file does not exist but we know prior
to it what is the disk type so in this case the disk type should be read
from a parameter.

So only when no value for disk type is passed, we get the disk type according
to source path.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Set guest disk cache to none to support live migration

According to commit 707aee09 the configuration "cache=none" must be set
to allow live migration.
So do it on get_disk_xml() to make sure all guest disks will have this
configuration.

The test needed to be updated because libvirt uses O_DIRECT flag when
cache=none to read the disk file and it is not supported on tmpfs.

For reference: https://www.redhat.com/archives/libvir-list/2013-August/msg00783.html

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Change VMTemplate._get_scsi_disks_xml() to use the common get_disk_xml()

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Change VMTemplate._get_iscsi_disks_xml() to use the common get_disk_xml()

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Change VMTemplate._get_disks_xml() to use the common get_disk_xml()

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Change VMTemplate._get_disks_xml() to handle all type of disks

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Remove VMTemplate._get_scsi_disks_xml() and VMTemplate._get_iscsi_disks_xml()

VMTemplate._get_disks_xml() handles all type of disks so those
functions are not needed anymore.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>