Add flags for virDomainOpenConsole

Add flags for virDomainOpenConsole

This patch adds a set of flags to be used with the virDomainOpenConsole
API call to specify if the user wishes to interrupt an existing console
session or just to try open a new one.

VIR_DOMAIN_CONSOLE_SAFE - specifies that the console connection should
                          be opened only if the hypervisor supports
                          mutually exclusive access to console devices

VIR_DOMAIN_CONSOLE_FORCE - specifies that the caller wishes to interrupt
                           existing session and force a creation of a
                           new one.