[Linux] Increase video memory to 8MB for Ubuntu desktop VM

[keirazhang]

Default video memory is only 4 MB, which could cause Ubuntu failed to launch the display manager. Increase it to 8MB.

+-------------------------------------------------------------------------+
| GuestInfo Detailed Data   | architecture='X86'                          |
|                           | bitness='64'                                |
|                           | distroAddlVersion='23.10 (Mantic Minotaur)' |
|                           | distroName='Ubuntu'                         |
|                           | distroVersion='23.10'                       |
|                           | familyName='Linux'                          |
|                           | kernelVersion='6.5.0-26-generic'            |
|                           | prettyName='Ubuntu 23.10'                   |
+-------------------------------------------------------------------------+


Test Results (Total: 3, Passed: 3, Elapsed Time: 00:35:12)
+-------------------------------------------------------------+
| ID | Name                              | Status | Exec Time |
+-------------------------------------------------------------+
|  1 | deploy_vm_efi_paravirtual_vmxnet3 | Passed | 00:24:14  |
|  2 | ovt_verify_install                | Passed | 00:07:33  |
|  3 | ovt_verify_status                 | Passed | 00:03:05  |
+-------------------------------------------------------------+