qemu: Introduce qemuBuildMasterKeyCommandLine

qemu: Introduce qemuBuildMasterKeyCommandLine

If the -object secret capability exists, then get the path to the
masterKey file and provide that to qemu. Checking for the existence
of the file before passing to qemu could be done, but causes issues
in mock test environment.

Since the qemuDomainObjPrivate is not available when building the
command line, the qemuBuildHasMasterKey API will have to suffice
as the primary arbiter for whether the capability exists in order
to find/return the path to the master key for usage.

Created the qemuDomainGetMasterKeyAlias API which will be used by
later patches to define the 'keyid' (eg, masterKey) to be used by
other secrets to provide the id to qemu for the master key.