virsh: remove driver check from attach-disk command

Virsh shouldn't check for driver support but rather let the backend handled this.
After removing the check, I can successfully attach file-based images to a qemu
VM with attach-disk.

% virsh attach-disk vm2 /images/test02.img vdc --driver qemu --type disk --subdriver raw
Disk attached successfully

This command generates the following XML:

<disk type='block' device='disk'>
      <driver name='qemu' type='raw'/>
      <source dev='/images/test02.img'/>
      <target dev='vdc' bus='virtio'/>
      <alias name='virtio-disk2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
</disk>
Signed-off-by: NRyan Harper <ryanh@us.ibm.com>

tools/virsh.c

@@ -8146,13 +8146,8 @@ cmdAttachDisk(vshControl *ctl, const vshCmd *cmd)
     mode = vshCommandOptString(cmd, "mode", NULL);
 
     if (driver) {
-        if (STREQ(driver, "file") || STREQ(driver, "tap")) {
+        if (STREQ(driver, "file") || STREQ(driver, "tap"))
             isFile = 1;
-        } else if (STRNEQ(driver, "phy")) {
-            vshError(ctl, _("No support for %s in command 'attach-disk'"),
-                     driver);
-            goto cleanup;
-        }
     }
 
     if (mode) {