add VirtFail resource and use it in new regression tests

add VirtFail resource and use it in new regression tests

This resource let us test a defined failiure behaviour ofi services.

Through the VMID we define how it should behave, with the folowing
rules:

When the service has the SID "fa:abcde" the digits a - e mean:

a - no meaning but can be used for differentiating similar resources
b - how many tries are needed to start correctly (0=default)
c - how many tries are needed to migrate correctly (0=default)
d - should shutdown be successful (0 = yes, anything else no)
e - return value of $plugin->exists() defaults to 1 if not set

a,b,c should always be set even if b and c have defaults (makes test
purpose clearer)
d and e

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>