The mode in which this disk is attached to the source instance,
either READ_WRITE or READ_ONLY.
Values:
UNDEFINED_MODE (0):
A value indicating that the enum field is not
set.
READ_ONLY (91950261):
Attaches this disk in read-only mode.
Multiple virtual machines can use a disk in
read-only mode at a time.
READ_WRITE (173607894):
[Default] Attaches this disk in read-write mode. Only one
virtual machine at a time can be attached to a disk in
read-write mode.