Importing VM: check ovf sha1 checksum before vmdk's one

[Tsso]

Having the ovf and vmdk files, when importing the two files and checksum is verified as stated in the .mf file, check first the ovf file because has much less size, and prevents wastes of time in case the ovf has been modified to workaround missconfigurations.

[Frank Mehnert]

We will implement that eventually. However, as the normal case is that the checksum is correct, the problem you want to address is quite low-priority.